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An electric-field assisted growth control methodology for integrating ZnO nanorods with microstructures† X. Zong and R. Zhu* The growth control of ZnO nanorods bridging over two microelectrodes in a three-electrode structure (the top cathode and anode, and the bottom gate) was realized using a wet chemical method with the assistance of an electric field generated by applying AC sine wave power on the top electrodes and a DC voltage on the bottom gate. A numerical control model for controlling the growth position, direction and density of ZnO nanorods on the microstructure was established based on the simulation of the electricfield distribution around the microstructures. The three input parameters in the numerical control model were defined as the peak-to-peak voltage of the AC sine wave (x1), the frequency of the AC sine wave (x2) and gate voltage (x3). Moreover, five output parameters (y1, y2, y3, y4, y5) in the model were defined as the electric field intensities at specific points on the electrodes to characterize the growth rate, direction, position and morphology of the ZnO nanorods integrated with the microelectrodes. The relationship
Received 10th June 2014, Accepted 15th August 2014
between the defined outputs and inputs were established using 3rd polynomial fitting, which served as the
DOI: 10.1039/c4nr03184a
numerical control model for the prediction of nanorod growth. The experimental results validated that growth control methodology provides us with an effective approach to integrate ZnO nanorods into
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devices.
Introduction Owing to their wide band gap energy (3.3 eV), high electron binding energy (60 meV), high electron mobility and large piezoelectric coefficient,1,2 one-dimensional ZnO nanorods (NRs) and nanowires (NWs) have attracted a lot of interest as promising materials with broad applications in piezoelectric nanogenerators,3,4 UV sensors,5–7 light-emitting diodes,8 solar cells9,10 and so on. The growth techniques of ZnO nanorods and nanowires can be mainly classified into two categories: vapor-phase methods and wet chemical methods. The vaporphase method, which includes vapor–solid (VS),11 vapor– liquid–solid (VLS),12–14 and chemical vapor deposition (CVD),15–17 is characterized with high quality and multiple morphologies. However, it cannot be compatible with most of micro-machined structures (such as micro metallic electrodes), which could not bear the high process temperatures (400 °C to 1500 °C) used during vapor-phase deposition. In order to collect functional signals of ZnO NRs or NWs, an additional
State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing 100084, China. E-mail: [email protected]; Fax: +86 10 62788935; Tel: +86 10 62788935 † Electronic supplementary information (ESI) available: Magnified graph of Fig. 4. See DOI: 10.1039/c4nr03184a
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fabrication process is needed to construct electrodes on the nanomaterials or an assembly process is necessary to place the pre-synthesized nanomaterials on and between two preprepared electrodes.4,6,7,18 Compared with vapor-phase methods, the wet chemical method has the merits of low temperature (below 100 °C), low cost and compatibility with micromachined structures.19,20 Therefore, it provides more choices for substrates and enables mass production. In practical device integration, fabrication reliability and compatibility with existing techniques are very important. The advantages of wet chemical methods can easily meet the requirements of cost and compatibility. However, the control over the growth position and direction of ZnO nanomaterials on microstructures remains a technological challenge.5,21 In order to control the position, direction and morphology of ZnO nanomaterials, the selective area method has been widely studied such as using prepatterned substrates, including a self-assembled monolayer of submicron spheres,22 laser interference lithography23 and electron-beam lithography.24,25 Moreover, the hydrothermal local selective growth method of ZnO NRs at low temperature has also attracted a lot of interest with applications such as laser local heating,26 selective acetate thermal decomposition,27 inkjet-printed seed particles,28 inkjet-printed zinc acetate29 and micro-contact printed seed particles.30 In this study, we propose a non-patterned
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method to meet the requirement of position selective and morphology control using a wet chemical method with assistance of an applied electric-field. Seedless growth of ZnO NRs bridging over the opposite ends of two microelectrodes is realized, which is a typical structure of nano-sensors. The tailordesigned electric-field in the hydrothermal solution enables control over the growth position and direction of ZnO nanorods. Using the wet chemical method, growth control over the density, length and diameter of the ZnO nanowires by solution concentration and temperature have been realized.31,32 In this article, the experimental results demonstrate that the density, length and diameter of ZnO nanorods could also be controlled by the applied electrode-field during the growth process. In order to determine the optimum growth conditions, a numerical control model was established based on simulation results. The simulation model, which considers the electric doublelayer capacitances on the liquid–solid interfaces between the solution and metallic electrodes was constructed to simulate the electric field around the electrodes using Comsol software. The peak-to-peak voltage of the AC sine wave (x1), the frequency of the AC sine wave (x2) and gate voltage (x3) were defined as input parameters, and the electric-field intensities (y1, y2, y3, y4, y5) at five specific points on the electrodes were defined as output parameters indicating the growth direction, position and morphology of the ZnO nanorods growing on and between the microelectrodes. Based on the simulation experiments, the relationship between the defined outputs and inputs were built using 3rd polynomial fitting, which served as the numerical control model. Finally, the well-controlled growth of ZnO nanorods bridging over the opposite ends of the cathode and anode electrodes was realized using control model prediction. The establishment of the numerical control model is of great importance for the control of ZnO NRs integration with microdevices.
Experimental Preparation of the three-electrode microstructure The fabrication process of the three-electrode microstructure started with the preparation of an N-doped silicon wafer with a low resistivity of 0.008–0.02 Ω cm. Then, a 500 nm SiO2 insulation layer was formed on the surface of the silicon wafer by thermal oxidation. Next, the SiO2 layer was masked by photoresist and exposed to form the patterned photoresist film followed by wet etching to expose the N-doped silicon wafer (to serve as a gate). Afterwards, a 100 nm Cr/Au film was deposited and patterned using photolithography to form a comb-like cathode and anode with a gap distance of 3 μm. Finally, wire bonding was used to connect the three electrodes with outside pins. Growth of ZnO NRs The schematic drawing of the fabrication method using a wet chemical method combined with electric-field assistance is shown in Fig. 1(a). A similar field effect transistor (FET)
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Fig. 1 (a) A schematic drawing of the growth of ZnO nanorods. The two top electrodes are defined as the cathode and anode. The bottom silicon wafer with low resistivity (0.008–0.02 Ω cm) is defined as the gate. (b) A schematic view of the desired morphology of ZnO NRs growing on and between the cathode and anode. The inset in (b) is the higher resolution image of the ZnO NRs.
structure with three microelectrodes, including two top coplanar Au electrodes (cathode and anode) and a bottom electrode (gate), was pre-prepared using micromachining technology. ZnO NRs were grown using a wet chemical method with the assistance of an applied electric-field. The whole sample was immerged in an equal molar aqueous solution (0.015 M) of Zn(NO3)2·6H2O and HMTA (C6H12N4) heating at 75 °C with the target surface side facing down. The electric signal was applied through the pins on the back side. An AC sine-wave voltage (0.5–1 MHz, 3.6–4.5 Vpp, generated by a Tektronix Arbitrary/ Function Generator AFG 3252) was applied on the anode and the cathode was connected to the ground. The gate electrode was connected to a DC voltage from 40 mV to −80 mV (generated by a DC power supply DH1715 A-3). After 3.5 hours of the heating process, the chip was picked up from the solution, rinsed with deionized water using ultrasonic cleaning and dried in air. Fig. 1(b) is a schematic view of the desired morphology of ZnO NRs growing locally on the opposite ends of the top two electrodes (cathode and anode) under these processing conditions.
Material characterization and electric measurement The structural properties of the ZnO NRs were characterized using SEM (JSM-6460LV and CSM-950) with energy-dispersive X-ray spectroscopy (EDX) and HRTEM (FEI TECNAI G2 F20 at 200 kV) with SAED pattern. The current–voltage (I–V) characteristics of the electrode–NRs–electrode device were measured using an Electrometer/High Resistance Meter (Keithley 6517B). The UV photoresponse of the device was conducted using a 1.26 mW, 365 nm LED (LLS-365, Ocean optics) with a 10 nm FWHM in air at room temperature. The thermal sensitivity of
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the device was measured in a temperature-controlled oven (CIMO DZF-6020).
Results and discussion Growth results of the ZnO nanorods Fig. 2 shows the growth results of the ZnO nanorods under different conditions. The experimental results in Fig. 2(a)–(h) demonstrate that the growth position and density of the ZnO NRs are greatly influenced by the gate voltage and the anode sine-wave frequency. In Fig. 2(a), (c) and (e), ZnO NRs were grown under the same sine wave (3.6 Vpp and 1 MHz) with different gate voltages (40 mV in (a)–(b); GND in (c)–(d); −40 mV in (e)–(f )). As shown in Fig. 2(a) and (b), when a 40 mV voltage was applied to the gate, the growth area of the ZnO NRs covered all the surface of cathode and anode. The ZnO NRs grew densely on the opposite ends of the electrodes and crossed connection between the two electrode ends. When the gate was connected to GND in the process of growth, the ZnO NRs grown also covered all the surface of the cathode and anode, but the density decreased as shown in Fig. 2(c) and (d). Furthermore, changing the gate voltage to −40 mV (in Fig. 2(e) and (f )), we found that the growth of the ZnO NRs was greatly restrained. The growth area of ZnO NRs shrank to the opposite end areas of the comb-finger electrodes. The lengths of ZnO
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NRs were also shortened greatly. When the gate voltage was further changed to −80 mV, no ZnO NRs were grown on the cathode and anode, which is not shown in Fig. 2. These experimental results demonstrate that the growth position and density of the ZnO NRs can be controlled by the gate voltage. In addition to the gate voltage, the sine-wave frequency of the anode voltage was found to be another factor that can be used to control the growth of NRs. Compared with Fig. 2(e) under the frequency of 1 MHz, the frequency of the sine wave in Fig. 2(g) was decreased to 0.5 MHz. At a negative gate voltage (−40 mV) and a lower frequency, the growth position of the NRs was concentrated to the opposite ends of cathode and anode, and the ZnO NRs grew more densely and crossed connection between the two electrode ends. This demonstrates the enhancement effect of a lower frequency on the growth of the ZnO NRs. Fig. 3(a) shows the SEM image of the ZnO NRs growing on the ends of the microelectrodes with a diameter of 100–600 nm, a length of 1–3 μm and a hexagonal cross section of the NRs. The SEM image of the ZnO NR junctions between the two electrodes is shown in Fig. 3(b). The EDX (Fig. 3(c)) results of the ZnO NRs show the atomic ratio of zinc to oxygen is near a stoichiometric composition (1 : 1), which demonstrates the growth of ZnO. To characterize the materials structure, TEM testing was conducted. Fig. 3(d) shows the TEM image of the ZnO NRs and the corresponding selected area
Fig. 2 SEM images of the ZnO NRs grown on microelectrodes. (b), (d), (f), (h), ( j) and (l) are high resolution SEM images of (a), (c), (e), (g), (i) and (k) respectively. ZnO NRs were grown under 3.6 Vpp ( peak-to-peak voltage), 1 MHz (frequency) sine-wave electric field with different gate voltages (40 mV in (a), GND in (c) and −40 mV in (e)). SEM image of the ZnO NRs grown under (g) 3.6 Vpp, 0.5 MHz sine wave with −40 mV gate voltage, (i) 4.5 Vpp, 1 MHz sine wave with −40 mV gate voltage and (k) 3.8 Vpp, 1 MHz sine wave with −30 mV gate voltage.
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Fig. 3 (a) SEM image of the ZnO NRs showing the hexagonal cross section. (b) SEM image showing the ZnO NR junctions. (c) EDX spectrum of a single ZnO NR on SEM and its elemental analysis (inset). (d) TEM image of the ZnO NRs and (e) SAED pattern. (f ) HRTEM image of the ZnO NRs.
electron diffraction (SAED) pattern (Fig. 3(e)), which indicates the single crystal structure of ZnO NRs and their growth direction. HRTEM examination (Fig. 3(f )) further proves the single-crystal structure of the ZnO NRs. Establishment of growth control methodology To better understand the growth mechanism of NRs, a simulation model was established and a simulation of the electric field on the surface of the microelectrodes was conducted using Comsol Multiphysics software. The coordinate system is defined in Fig. 4(a) and a schematic diagram of the 2-D model in the x–y plane is shown in Fig. 4(c). The simulation chip was immerged in a solution environment with a conductivity of 50 S cm−1, which was measured in the solution of zinc nitrate
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and HMTA using an impedance analyser. Based on surface impedance analysis, the equivalent surface resistance and capacitance were set at 200 Ω m2 and 0.4 F m−2, respectively. The gap distance between the cathode and anode was 3 μm. Fig. 4(a), (c), (e) and (g) show the y components of the electric field intensity (Ey) at 20 equispaced time points in one period under different gate voltages and frequencies. Fig. 4(b), (d), (f ) and (h) show the x components of the electric-field intensity (Ex) corresponding to Fig. 4(a), (c), (e) and (g). In Fig. 4(a)–(f ), the electric field intensities were calculated under the same sine wave (3.6 Vpp and 1 MHz) and at different gate voltages (40 mV in (a)–(b); GND in (c)–(d); −40 mV in (e)–(f )). In Fig. 4(g) and (h) the sine wave frequency was set at 0.5 MHz and the electric field intensities were calculated under sine wave (3.6 Vpp and 0.5 MHz) and gate voltage of −40 mV. In the hydrothermal synthesis of ZnO nanowires, Zn(NO3)2 and HMTA were used.33 In the case without an applied electric field, Zn(NO3)2 provides Zn2+ ions and H2O molecules provide OH− ions for building up the ZnO nanowires in the solution. As the reaction proceeds slowly at room temperature, the solution is heated at 75 °C to improve the growth rate. According to previous reports,34 with an applied electricfield to provide electrons, the reaction of ZnO nanorods can be formulated as follows: NO3 þ H2 O þ 2e ! NO2 þ 2OH
ð1Þ
Zn2þ þ 2OH ! ZnO þ H2 O
ð2Þ
The reaction equations show that with an applied electricfields, an increase in OH− concentration induced by the reaction of NO3− with H2O will occur on the negative electrode due to the participation of electrons. Thus, the growth of ZnO NRs will be promoted. Under an AC electric field, the direction and
Fig. 4 Simulation results of the y and x components of the electric field vector. (a), (c), (e) and (g) show the y components of the electric field intensity (Ey) under different conditions. (b), (d), (f ) and (h) show the x components of the electric field intensity (Ex) (corresponding to (a), (c), (e) and (g)). The coordinate system is defined in (a) and a schematic of the 2-D model in the x–y plane is shown in (c). The definition of output parameters (y1, y2, y3, y4, y5) are illustrated in (a) and (b). The legend on the right side shows the specific time in one period (T) for each color.
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magnitude of the electric field vector can be regulated using the gate voltage. For the y component of the electric field (Ey) on the microelectrode, a negative value of Ey indicates that the direction of the electric field points to the interior of the electrode, which means the Au electrode surface tends to provide free electrons to the solution of Zn(NO3)2 and HMTA and promotes ZnO NRs growth. Conversely, a positive value of Ey indicates that the direction of the electric field points to the exterior of the electrode, which will inhibit the growth of the ZnO NRs on the electrodes without a supply of electrons. For the x component of the electric field (Ex) at the end of the microelectrode, a negative value indicates promotion of the lateral growth of ZnO NRs on the anode (left electrode) but restraint of the lateral growth of ZnO NRs on the cathode (right electrode). Conversely, a positive value of Ex indicates promotion of the lateral growth of ZnO NRs on the cathode (right electrode) but restraint of the lateral growth of ZnO NRs on the anode (left electrode). To establish the numerical control model, shown in Fig. 4(a) and (b) five output parameters were defined as follows: y1 – the negative peak value of the y component of the electric field at the end of the left anode; y2 – the negative peak value of the y component of the electric field at the end of the right cathode; y3 – the minimum value of the y component of the electric field on the electrode surfaces away from the ends; y4 – the negative peak value of the x component of the electric field at the end of the left anode; y5 – the positive peak value of the x component of the electric field at the end of the right cathode. The outputs y1 and y2 characterize the vertical growth of the ZnO NRs growing along the y axis at the ends of anode and cathode, respectively. The parameter y3 characterizes the growth of the ZnO NRs on the anode and cathode surface away from the ends. The parameters y4 and y5 characterize the lateral growth of the ZnO NRs growing along the x axis at the ends of anode and cathode, respectively. In Fig. 4(a)–(f ), with gate voltage changing from 40 mV to GND and further to −40 mV, the magnitudes of the parameters y1 and y2 were decreased, which implies a lowered growth for the ZnO NRs growing in a vertical direction at the electrode ends. The parameter y3 varied from a negative value to positive value, which implies the absence of free electrons supplied for ZnO NRs growth. The magnitude of the parameters y4 and y5 were also decreased, which implies a lowered growth for the ZnO NRs growing in a lateral direction at the electrode ends. The variations of the parameters (y1, y2, y3, y4, y5) can explain the growth of the ZnO NRs under different gate voltage, which is in accordance with the experimental growth of the ZnO NRs shown in Fig. 2. As shown in Fig. 4(e)–(h), when decreasing the frequency of the anode sine wave from 1 MHz to 0.5 MHz, the magnitude of parameters y1, y2, y4 and y5 became larger and y3 remained positive, which implies the enhanced growth of the ZnO NRs on the ends of electrodes and inhibition on the surface of electrodes, which is in accordance with the experimental results in Fig. 2(e)–(h).
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Based on the above analysis, we can deduce that the parameters (y1, y2, y3, y4, y5) characterize the growth of ZnO NRs on the electrodes including growth position, direction and density. These parameters are completely dependent on the AC sine wave on the anode and the DC gate voltage. The relationship model between the parameters (y1, y2, y3, y4, y5) and the applied AC sine wave and gate voltage can be established using a numerical polynomial control model for controlling and predicting the growth of ZnO NRs. The parameters (y1, y2, y3, y4, y5) are defined as outputs in the control model. The three inputs of the model were defined as follows: x1 – the peak-to-peak voltage of the AC sine wave on the anode; x2 – the frequency of the AC sine wave on the anode; x3 – the DC gate voltage. The analytical expression of the kth order multivariate polynomial can be written as follows: yj ¼
kab ka X k X X c¼0
j
pa;b;c xa1 xb2 xc3 ; j ¼ 1; 2; 3; 4; 5
ð3Þ
b¼0 a¼0
where j is the subscript of the five outputs (y1, y2, y3, y4, y5); x1, x2 and x3 are the inputs; a, b and c represent the degrees of inputs x1, x2 and x3, respectively; pja,b,c is the coefficient determined by fitting. Table 1 shows the values of x1, x2 and x3, which contains 24 different conditions. Using the simulation model, the outputs (y1, y2, y3, y4, y5) corresponding to the inputs (x1, x2, x3) listed in Table 1 were calculated in COMSOL. Afterwards, the relationship model between the outputs and the inputs were established using polynomial fitting based on the least square method. In order to evaluate the fitting accuracy of the polynomial model, the root-mean-square errors (RMSE) of polynomial model were analyzed. Fig. 5 shows the errors versus the polynomial order (from 1st to 4th). As we can see, the RMSE
Table 1
Values of x1, x2 and x3
x1 (Vpp)
x2 (MHz)
x3 (mV)
2.0/3.0/4.0/5.0
0.5/1
0/−40/−80
Fig. 5 The RMSE of the five outputs (y1, y2, y3, y4, y5) and average value versus the polynomial order.
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decreased dramatically from 1st to 2nd order. However, with a further increase in the polynomial order, the RMSE decreased slightly. Considering that a higher polynomial order will increase the computation complexity, we finally selected the 3rd order polynomial model as the numerical control model. The above analyses demonstrate that the magnitudes of y1 and y2 represent the vertical growth of the ZnO NRs on the ends of the anode and cathode, respectively. In addition, the magnitudes of the outputs y4 and y5 represent the lateral growth of the ZnO NRs on the ends of the anode and cathode, respectively. The increase in the magnitudes of y1, y2, y4 and y5 implies the electrode ends can provide more electrons. Under this condition, the reaction of NO3− ions with H2O can be greatly accelerated, and thus results in an increase in the OH− concentration and promotes the growth of the ZnO NRs. Output y3 characterizes the growth of the ZnO NRs on the electrode surface away from the ends. A negative value of y3 implies the supply of free electrons and the promotion of ZnO NRs growth, while a positive value of y3 implies the restraint of growth of the ZnO NRs. The increases in x1 and x3 will enhance the magnitude of all outputs, accelerate the reaction of NO3− ions with H2O and promote the growth of the ZnO NRs. However, due to the effect of surface capacitance, input x2 plays an opposite role. Specifically, decreasing the value of x2 will enhance the magnitude of all outputs, accelerate the reaction of NO3− ions with H2O and thus promote the growth of the ZnO NRs.
were located on the opposite ends of the anode and cathode. Fig. 2(k) and (l) show another example under the conditions of 3.8 Vpp, 1 MHz sine wave and −30 mV gate voltage. The outputs calculated using the model are −251.89 V m−1, −241.30 V m−1, 1.07 V m−1, −334.21 V m−1, 330.36 V m−1, which indicates a weaker growth of the NRs on the electrode ends compared with the conditions in Fig. 2(i) and ( j). These experiments further validated the effectiveness of prediction using the 3rd polynomial numerical control model. Fig. 6 shows the control of the morphology of the ZnO NRs growing on and between the two opposite ends of the cathode and anode by different gate voltage, AC sine-wave peak-to-peak value and frequency. In Fig. 6(a)–(c), the ZnO NRs were grown under the same sine-wave (3.6 Vpp, 1 MHz) with different gate voltage varying from −40 mV to GND and further to 40 mV, the average diameter of the ZnO NRs increased from 170 nm to
Validation of the prediction model Table 2 shows the output values calculated using the established 3rd polynomial model corresponding to the experimental growth conditions in Fig. 2(a)–(h). Furthermore, using the numerical control model, we calculated the outputs (y1, y2, y3, y4, y5) under the conditions of 4.5 Vpp, 1 MHz sine wave of anode with a −40 mV gate voltage applied. The calculated outputs (−277.58 V m−1, −260.73 V m−1, 20.64 V m−1, −385.32 V m−1, 382.97 V m−1) are also shown in Table 2, which indicates a strong growth of NRs on the electrode ends (due to the larger magnitudes of y1, y2, y3, y5), and an inhibitory effect on the NRs growth on electrode surface (due to a higher positive value of y3). Fig. 2(i) and ( j) show the SEM images of ZnO NRs grown under the conditions of 4.5 Vpp, 1 MHz sine wave of anode voltage and −40 mV gate voltage. The ZnO NRs grown
Fig. 6 SEM images of the morphologies of the ZnO NRs growing on and between the two opposite ends of the cathode and anode. The growth conditions are shown on the top of each image.
Table 2 The output values of the 3rd polynomial model and the average diameter and length of the ZnO NRs growing between the anode and cathode with different input values
x1 (Vpp)
x2 (MHz)
x3 (mV)
y1 (V m−1)
y2 (V m−1)
y3 (V m−1)
y4 (V m−1)
y5 (V m−1)
Ēxa (V m−1)
Db (nm)
Lc (μm)
3.6 3.6 3.6 3.6 4.5 3.8
1 1 1 0.5 1 1
40 0 −40 −40 −40 −30
−371.14 −288.59 −204.35 −332.32 −277.58 −251.89
−408.07 −302.43 −187.16 −283.55 −260.73 −241.30
−70.04 −5.65 19.80 5.24 20.64 1.07
−488.87 −392.36 −286.12 −455.05 −385.32 −334.21
526.40 411.95 282.53 402.69 382.97 330.36
507.64 402.16 284.33 428.87 384.15 332.29
530 460 170 480 430 360
2.7 2.2 0.65 2.4 2.1 1.7
Ēx is the average value of |y4| and |y5|. b D is the average diameter of the ZnO NRs laterally grown between the anode and cathode. c L is the average length of the ZnO NRs laterally grown between the anode and cathode.
a
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460 nm and further to 530 nm, the average length of NRs increased from 0.65 μm to 2.2 μm and further to 2.7 μm and the density were also gradually increased. Comparison of the growth results with different peak-to-peak value of the sinewave on the anode are shown in Fig. 6(a) and (d). With an increase of peak-to-peak value from 3.6 Vpp to 4.5 Vpp, the average diameter of the ZnO NRs increased from 170 nm to 430 nm, the average length increased from 0.65 μm to 2.1 μm and the density also increased. The growth morphology of NRs (360 nm, 1.7 μm) in Fig. 6(e) fell between Fig. 6(a) and (b), also between Fig. 6(a) and (d), due to the moderate gate voltage and anode sine-wave voltage. Fig. 6(a) and (f ) were conducted with the same gate voltage (−40 mV) and peak-to-peak value (3.6 Vpp) but different frequencies (1 MHz in (a) and 0.5 MHz
Fig. 7 (a) The relationship between Ēx (the average value of |y4| and |y5|) and D (the average diameter of the ZnO NRs). (b) the relationship between Ēx and L (the average length of the ZnO NRs). The values in brackets indicate the corresponding values of (x1, x2, x3).
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in (f )). Further comparison of Fig. 6(a) and (f ) shows that with a decrease of frequency, the average diameter of the ZnO NRs increased from 170 nm to 480 nm, the length increased from 0.65 μm to 2.4 μm and the density significantly increased. The results indicate that the gate voltage and the AC sine-wave on the anode could greatly dominate the diameter, length and density of ZnO NRs growing on and between the opposite ends of the two microelectrodes. As analysed above, the magnitudes of y1 and y2 determine the vertical growth of the ZnO NRs on the electrode ends, while the magnitudes of y4 and y5 determine the lateral growth of the ZnO NRs between the electrode ends. Considering the lateral growth of the ZnO NRs, the average value of |y4| and |y5| (represented by Ēx) and the average values of diameter (D) and length (L) of the NRs laterally grown between the anode and cathode are listed in Table 2. Fig. 7(a) and (b) reveal the positive correlation between Ēx (the average value of |y4| and |y5|) and D and L. As shown in Fig. 4 and Table 2, the magnitudes of y1 and y2 as well as the magnitudes of y4 and y5 are not strictly identical, which may result in slightly different formation of ZnO NRs on the anode and cathode. Taking Fig. 6(a) for example, the values of (y1, y2, y4, y5) are −204.35, −187.16, −286.12, 282.53, respectively and are listed in Table 2. The magnitude of y1 is larger than y2 and the magnitude of y4 is a little larger than y5. The results shown in Fig. 6(a) demonstrate that the ZnO NRs grew more densely on the left anode than the cathode, which is in accordance with the theoretical computations. The device chips with ZnO NRs grown were further characterized by properties such as electricity, photoelectricity, and thermoelectricity. The primary results are shown in Fig. 8. Fig. 8(a) shows the IV characteristics, which demonstrates the Schottky contacts between the ZnO NRs and Au electrodes. Fig. 8(b) shows the UV photoresponse of the device under a 365 nm LED (1.26 mW) in air at room temperature. The device shows the rise time (to 63%) of about 0.5 s and decay time (to 37%) of about 1 s with the current rising from 0.12 μA to 2.92 μA by 2333% with the applied bias of 3 V. Fig. 8(c) shows the negative temperature coefficient (NTC) characteristics of the ZnO NRs device. In virtue of a high surface-to-volume ratio by overhanging structure, the ZnO NRs device possesses an
Fig. 8 Various properties of the ZnO NRs chip. (a) I–V characteristics, (b) UV photoresponse under the illumination of a 365 nm UV LED with an intensity of 1.26 mW, (c) thermal sensitivity, RT = R0 exp(B/T ), where T is value of absolute temperature, RT is the resistance corresponding to the working temperature T, R0 = 4.67 Ω is the resistance when T → ∞ and B = 1926.4 K is the thermal constant.
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excellent heat exchange with ambient medium, which allows a good potential in ultra-fast thermal response and this work is in progress. All these experiments demonstrate the promising potentials for versatile sensor applications.
Conclusions In this study, an electric-field assisted hydrothermal growth method for integrating ZnO NRs with microstructures is proposed. By using this method, the growth position, direction and morphology of ZnO NRs can be easily controlled. A 3rd polynomial control model with the inputs ( peak-to-peak value and frequency of the AC sine wave on the anode, and gate voltage) and the outputs representing the growth position, direction and morphology of the ZnO NRs was established, which can be used for the prediction of the ZnO NRs growth. The electric field control methodology is of great importance for the control of ZnO NRs growth and makes it feasible to integrate ZnO NRs with microdevices. The sensing properties of the ZnO NRs demonstrate promising potential for versatile sensor applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) under grant no. 91123017 and the National Key Project of Scientific Instrument and Equipment Development under grant no. 2012YQ030261.
Notes and references 1 Z. L. Wang, J. Phys.: Condens. Matter, 2004, 16, R829–R858. 2 G. C. Yi, C. R. Wang and W. I. Park, Semicond. Sci. Technol., 2005, 20, S22–S34. 3 X. D. Wang, J. H. Song, J. Liu and Z. L. Wang, Science, 2007, 316, 102–105. 4 R. S. Yang, Y. Qin, L. M. Dai and Z. L. Wang, Nat. Nanotechnol., 2009, 4, 34–39. 5 C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo and D. Wang, Nano Lett., 2007, 7, 1003–1009. 6 L. Guo, H. Zhang, D. X. Zhao, B. H. Li, Z. Z. Zhang, M. M. Jiang and D. Z. Shen, Sens. Actuators, B, 2012, 166, 12–16. 7 D. Z. Li and R. Zhu, Chin. Phys. B, 2013, 22, 018502. 8 W. I. Park and G. C. Yi, Adv. Mater., 2004, 16, 87–90. 9 G. J. Chang, S. Y. Lin and J. J. Wu, Nanoscale, 2014, 6, 1329–1334. 10 Y. G. Wei, C. Xu, S. Xu, C. Li, W. Z. Wu and Z. L. Wang, Nano Lett., 2010, 10, 2092–2096.
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An electric-field assisted growth control methodology for integrating ZnO nanorods with microstructures† X. Zong and R. Zhu* The growth control of ZnO nanorods bridging over two microelectrodes in a three-electrode structure (the top cathode and anode, and the bottom gate) was realized using a wet chemical method with the assistance of an electric field generated by applying AC sine wave power on the top electrodes and a DC voltage on the bottom gate. A numerical control model for controlling the growth position, direction and density of ZnO nanorods on the microstructure was established based on the simulation of the electricfield distribution around the microstructures. The three input parameters in the numerical control model were defined as the peak-to-peak voltage of the AC sine wave (x1), the frequency of the AC sine wave (x2) and gate voltage (x3). Moreover, five output parameters (y1, y2, y3, y4, y5) in the model were defined as the electric field intensities at specific points on the electrodes to characterize the growth rate, direction, position and morphology of the ZnO nanorods integrated with the microelectrodes. The relationship
Received 10th June 2014, Accepted 15th August 2014
between the defined outputs and inputs were established using 3rd polynomial fitting, which served as the
DOI: 10.1039/c4nr03184a
numerical control model for the prediction of nanorod growth. The experimental results validated that growth control methodology provides us with an effective approach to integrate ZnO nanorods into
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devices.
Introduction Owing to their wide band gap energy (3.3 eV), high electron binding energy (60 meV), high electron mobility and large piezoelectric coefficient,1,2 one-dimensional ZnO nanorods (NRs) and nanowires (NWs) have attracted a lot of interest as promising materials with broad applications in piezoelectric nanogenerators,3,4 UV sensors,5–7 light-emitting diodes,8 solar cells9,10 and so on. The growth techniques of ZnO nanorods and nanowires can be mainly classified into two categories: vapor-phase methods and wet chemical methods. The vaporphase method, which includes vapor–solid (VS),11 vapor– liquid–solid (VLS),12–14 and chemical vapor deposition (CVD),15–17 is characterized with high quality and multiple morphologies. However, it cannot be compatible with most of micro-machined structures (such as micro metallic electrodes), which could not bear the high process temperatures (400 °C to 1500 °C) used during vapor-phase deposition. In order to collect functional signals of ZnO NRs or NWs, an additional
State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing 100084, China. E-mail: [email protected]; Fax: +86 10 62788935; Tel: +86 10 62788935 † Electronic supplementary information (ESI) available: Magnified graph of Fig. 4. See DOI: 10.1039/c4nr03184a
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fabrication process is needed to construct electrodes on the nanomaterials or an assembly process is necessary to place the pre-synthesized nanomaterials on and between two preprepared electrodes.4,6,7,18 Compared with vapor-phase methods, the wet chemical method has the merits of low temperature (below 100 °C), low cost and compatibility with micromachined structures.19,20 Therefore, it provides more choices for substrates and enables mass production. In practical device integration, fabrication reliability and compatibility with existing techniques are very important. The advantages of wet chemical methods can easily meet the requirements of cost and compatibility. However, the control over the growth position and direction of ZnO nanomaterials on microstructures remains a technological challenge.5,21 In order to control the position, direction and morphology of ZnO nanomaterials, the selective area method has been widely studied such as using prepatterned substrates, including a self-assembled monolayer of submicron spheres,22 laser interference lithography23 and electron-beam lithography.24,25 Moreover, the hydrothermal local selective growth method of ZnO NRs at low temperature has also attracted a lot of interest with applications such as laser local heating,26 selective acetate thermal decomposition,27 inkjet-printed seed particles,28 inkjet-printed zinc acetate29 and micro-contact printed seed particles.30 In this study, we propose a non-patterned
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method to meet the requirement of position selective and morphology control using a wet chemical method with assistance of an applied electric-field. Seedless growth of ZnO NRs bridging over the opposite ends of two microelectrodes is realized, which is a typical structure of nano-sensors. The tailordesigned electric-field in the hydrothermal solution enables control over the growth position and direction of ZnO nanorods. Using the wet chemical method, growth control over the density, length and diameter of the ZnO nanowires by solution concentration and temperature have been realized.31,32 In this article, the experimental results demonstrate that the density, length and diameter of ZnO nanorods could also be controlled by the applied electrode-field during the growth process. In order to determine the optimum growth conditions, a numerical control model was established based on simulation results. The simulation model, which considers the electric doublelayer capacitances on the liquid–solid interfaces between the solution and metallic electrodes was constructed to simulate the electric field around the electrodes using Comsol software. The peak-to-peak voltage of the AC sine wave (x1), the frequency of the AC sine wave (x2) and gate voltage (x3) were defined as input parameters, and the electric-field intensities (y1, y2, y3, y4, y5) at five specific points on the electrodes were defined as output parameters indicating the growth direction, position and morphology of the ZnO nanorods growing on and between the microelectrodes. Based on the simulation experiments, the relationship between the defined outputs and inputs were built using 3rd polynomial fitting, which served as the numerical control model. Finally, the well-controlled growth of ZnO nanorods bridging over the opposite ends of the cathode and anode electrodes was realized using control model prediction. The establishment of the numerical control model is of great importance for the control of ZnO NRs integration with microdevices.
Experimental Preparation of the three-electrode microstructure The fabrication process of the three-electrode microstructure started with the preparation of an N-doped silicon wafer with a low resistivity of 0.008–0.02 Ω cm. Then, a 500 nm SiO2 insulation layer was formed on the surface of the silicon wafer by thermal oxidation. Next, the SiO2 layer was masked by photoresist and exposed to form the patterned photoresist film followed by wet etching to expose the N-doped silicon wafer (to serve as a gate). Afterwards, a 100 nm Cr/Au film was deposited and patterned using photolithography to form a comb-like cathode and anode with a gap distance of 3 μm. Finally, wire bonding was used to connect the three electrodes with outside pins. Growth of ZnO NRs The schematic drawing of the fabrication method using a wet chemical method combined with electric-field assistance is shown in Fig. 1(a). A similar field effect transistor (FET)
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Fig. 1 (a) A schematic drawing of the growth of ZnO nanorods. The two top electrodes are defined as the cathode and anode. The bottom silicon wafer with low resistivity (0.008–0.02 Ω cm) is defined as the gate. (b) A schematic view of the desired morphology of ZnO NRs growing on and between the cathode and anode. The inset in (b) is the higher resolution image of the ZnO NRs.
structure with three microelectrodes, including two top coplanar Au electrodes (cathode and anode) and a bottom electrode (gate), was pre-prepared using micromachining technology. ZnO NRs were grown using a wet chemical method with the assistance of an applied electric-field. The whole sample was immerged in an equal molar aqueous solution (0.015 M) of Zn(NO3)2·6H2O and HMTA (C6H12N4) heating at 75 °C with the target surface side facing down. The electric signal was applied through the pins on the back side. An AC sine-wave voltage (0.5–1 MHz, 3.6–4.5 Vpp, generated by a Tektronix Arbitrary/ Function Generator AFG 3252) was applied on the anode and the cathode was connected to the ground. The gate electrode was connected to a DC voltage from 40 mV to −80 mV (generated by a DC power supply DH1715 A-3). After 3.5 hours of the heating process, the chip was picked up from the solution, rinsed with deionized water using ultrasonic cleaning and dried in air. Fig. 1(b) is a schematic view of the desired morphology of ZnO NRs growing locally on the opposite ends of the top two electrodes (cathode and anode) under these processing conditions.
Material characterization and electric measurement The structural properties of the ZnO NRs were characterized using SEM (JSM-6460LV and CSM-950) with energy-dispersive X-ray spectroscopy (EDX) and HRTEM (FEI TECNAI G2 F20 at 200 kV) with SAED pattern. The current–voltage (I–V) characteristics of the electrode–NRs–electrode device were measured using an Electrometer/High Resistance Meter (Keithley 6517B). The UV photoresponse of the device was conducted using a 1.26 mW, 365 nm LED (LLS-365, Ocean optics) with a 10 nm FWHM in air at room temperature. The thermal sensitivity of
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the device was measured in a temperature-controlled oven (CIMO DZF-6020).
Results and discussion Growth results of the ZnO nanorods Fig. 2 shows the growth results of the ZnO nanorods under different conditions. The experimental results in Fig. 2(a)–(h) demonstrate that the growth position and density of the ZnO NRs are greatly influenced by the gate voltage and the anode sine-wave frequency. In Fig. 2(a), (c) and (e), ZnO NRs were grown under the same sine wave (3.6 Vpp and 1 MHz) with different gate voltages (40 mV in (a)–(b); GND in (c)–(d); −40 mV in (e)–(f )). As shown in Fig. 2(a) and (b), when a 40 mV voltage was applied to the gate, the growth area of the ZnO NRs covered all the surface of cathode and anode. The ZnO NRs grew densely on the opposite ends of the electrodes and crossed connection between the two electrode ends. When the gate was connected to GND in the process of growth, the ZnO NRs grown also covered all the surface of the cathode and anode, but the density decreased as shown in Fig. 2(c) and (d). Furthermore, changing the gate voltage to −40 mV (in Fig. 2(e) and (f )), we found that the growth of the ZnO NRs was greatly restrained. The growth area of ZnO NRs shrank to the opposite end areas of the comb-finger electrodes. The lengths of ZnO
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NRs were also shortened greatly. When the gate voltage was further changed to −80 mV, no ZnO NRs were grown on the cathode and anode, which is not shown in Fig. 2. These experimental results demonstrate that the growth position and density of the ZnO NRs can be controlled by the gate voltage. In addition to the gate voltage, the sine-wave frequency of the anode voltage was found to be another factor that can be used to control the growth of NRs. Compared with Fig. 2(e) under the frequency of 1 MHz, the frequency of the sine wave in Fig. 2(g) was decreased to 0.5 MHz. At a negative gate voltage (−40 mV) and a lower frequency, the growth position of the NRs was concentrated to the opposite ends of cathode and anode, and the ZnO NRs grew more densely and crossed connection between the two electrode ends. This demonstrates the enhancement effect of a lower frequency on the growth of the ZnO NRs. Fig. 3(a) shows the SEM image of the ZnO NRs growing on the ends of the microelectrodes with a diameter of 100–600 nm, a length of 1–3 μm and a hexagonal cross section of the NRs. The SEM image of the ZnO NR junctions between the two electrodes is shown in Fig. 3(b). The EDX (Fig. 3(c)) results of the ZnO NRs show the atomic ratio of zinc to oxygen is near a stoichiometric composition (1 : 1), which demonstrates the growth of ZnO. To characterize the materials structure, TEM testing was conducted. Fig. 3(d) shows the TEM image of the ZnO NRs and the corresponding selected area
Fig. 2 SEM images of the ZnO NRs grown on microelectrodes. (b), (d), (f), (h), ( j) and (l) are high resolution SEM images of (a), (c), (e), (g), (i) and (k) respectively. ZnO NRs were grown under 3.6 Vpp ( peak-to-peak voltage), 1 MHz (frequency) sine-wave electric field with different gate voltages (40 mV in (a), GND in (c) and −40 mV in (e)). SEM image of the ZnO NRs grown under (g) 3.6 Vpp, 0.5 MHz sine wave with −40 mV gate voltage, (i) 4.5 Vpp, 1 MHz sine wave with −40 mV gate voltage and (k) 3.8 Vpp, 1 MHz sine wave with −30 mV gate voltage.
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Fig. 3 (a) SEM image of the ZnO NRs showing the hexagonal cross section. (b) SEM image showing the ZnO NR junctions. (c) EDX spectrum of a single ZnO NR on SEM and its elemental analysis (inset). (d) TEM image of the ZnO NRs and (e) SAED pattern. (f ) HRTEM image of the ZnO NRs.
electron diffraction (SAED) pattern (Fig. 3(e)), which indicates the single crystal structure of ZnO NRs and their growth direction. HRTEM examination (Fig. 3(f )) further proves the single-crystal structure of the ZnO NRs. Establishment of growth control methodology To better understand the growth mechanism of NRs, a simulation model was established and a simulation of the electric field on the surface of the microelectrodes was conducted using Comsol Multiphysics software. The coordinate system is defined in Fig. 4(a) and a schematic diagram of the 2-D model in the x–y plane is shown in Fig. 4(c). The simulation chip was immerged in a solution environment with a conductivity of 50 S cm−1, which was measured in the solution of zinc nitrate
Paper
and HMTA using an impedance analyser. Based on surface impedance analysis, the equivalent surface resistance and capacitance were set at 200 Ω m2 and 0.4 F m−2, respectively. The gap distance between the cathode and anode was 3 μm. Fig. 4(a), (c), (e) and (g) show the y components of the electric field intensity (Ey) at 20 equispaced time points in one period under different gate voltages and frequencies. Fig. 4(b), (d), (f ) and (h) show the x components of the electric-field intensity (Ex) corresponding to Fig. 4(a), (c), (e) and (g). In Fig. 4(a)–(f ), the electric field intensities were calculated under the same sine wave (3.6 Vpp and 1 MHz) and at different gate voltages (40 mV in (a)–(b); GND in (c)–(d); −40 mV in (e)–(f )). In Fig. 4(g) and (h) the sine wave frequency was set at 0.5 MHz and the electric field intensities were calculated under sine wave (3.6 Vpp and 0.5 MHz) and gate voltage of −40 mV. In the hydrothermal synthesis of ZnO nanowires, Zn(NO3)2 and HMTA were used.33 In the case without an applied electric field, Zn(NO3)2 provides Zn2+ ions and H2O molecules provide OH− ions for building up the ZnO nanowires in the solution. As the reaction proceeds slowly at room temperature, the solution is heated at 75 °C to improve the growth rate. According to previous reports,34 with an applied electricfield to provide electrons, the reaction of ZnO nanorods can be formulated as follows: NO3 þ H2 O þ 2e ! NO2 þ 2OH
ð1Þ
Zn2þ þ 2OH ! ZnO þ H2 O
ð2Þ
The reaction equations show that with an applied electricfields, an increase in OH− concentration induced by the reaction of NO3− with H2O will occur on the negative electrode due to the participation of electrons. Thus, the growth of ZnO NRs will be promoted. Under an AC electric field, the direction and
Fig. 4 Simulation results of the y and x components of the electric field vector. (a), (c), (e) and (g) show the y components of the electric field intensity (Ey) under different conditions. (b), (d), (f ) and (h) show the x components of the electric field intensity (Ex) (corresponding to (a), (c), (e) and (g)). The coordinate system is defined in (a) and a schematic of the 2-D model in the x–y plane is shown in (c). The definition of output parameters (y1, y2, y3, y4, y5) are illustrated in (a) and (b). The legend on the right side shows the specific time in one period (T) for each color.
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magnitude of the electric field vector can be regulated using the gate voltage. For the y component of the electric field (Ey) on the microelectrode, a negative value of Ey indicates that the direction of the electric field points to the interior of the electrode, which means the Au electrode surface tends to provide free electrons to the solution of Zn(NO3)2 and HMTA and promotes ZnO NRs growth. Conversely, a positive value of Ey indicates that the direction of the electric field points to the exterior of the electrode, which will inhibit the growth of the ZnO NRs on the electrodes without a supply of electrons. For the x component of the electric field (Ex) at the end of the microelectrode, a negative value indicates promotion of the lateral growth of ZnO NRs on the anode (left electrode) but restraint of the lateral growth of ZnO NRs on the cathode (right electrode). Conversely, a positive value of Ex indicates promotion of the lateral growth of ZnO NRs on the cathode (right electrode) but restraint of the lateral growth of ZnO NRs on the anode (left electrode). To establish the numerical control model, shown in Fig. 4(a) and (b) five output parameters were defined as follows: y1 – the negative peak value of the y component of the electric field at the end of the left anode; y2 – the negative peak value of the y component of the electric field at the end of the right cathode; y3 – the minimum value of the y component of the electric field on the electrode surfaces away from the ends; y4 – the negative peak value of the x component of the electric field at the end of the left anode; y5 – the positive peak value of the x component of the electric field at the end of the right cathode. The outputs y1 and y2 characterize the vertical growth of the ZnO NRs growing along the y axis at the ends of anode and cathode, respectively. The parameter y3 characterizes the growth of the ZnO NRs on the anode and cathode surface away from the ends. The parameters y4 and y5 characterize the lateral growth of the ZnO NRs growing along the x axis at the ends of anode and cathode, respectively. In Fig. 4(a)–(f ), with gate voltage changing from 40 mV to GND and further to −40 mV, the magnitudes of the parameters y1 and y2 were decreased, which implies a lowered growth for the ZnO NRs growing in a vertical direction at the electrode ends. The parameter y3 varied from a negative value to positive value, which implies the absence of free electrons supplied for ZnO NRs growth. The magnitude of the parameters y4 and y5 were also decreased, which implies a lowered growth for the ZnO NRs growing in a lateral direction at the electrode ends. The variations of the parameters (y1, y2, y3, y4, y5) can explain the growth of the ZnO NRs under different gate voltage, which is in accordance with the experimental growth of the ZnO NRs shown in Fig. 2. As shown in Fig. 4(e)–(h), when decreasing the frequency of the anode sine wave from 1 MHz to 0.5 MHz, the magnitude of parameters y1, y2, y4 and y5 became larger and y3 remained positive, which implies the enhanced growth of the ZnO NRs on the ends of electrodes and inhibition on the surface of electrodes, which is in accordance with the experimental results in Fig. 2(e)–(h).
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Based on the above analysis, we can deduce that the parameters (y1, y2, y3, y4, y5) characterize the growth of ZnO NRs on the electrodes including growth position, direction and density. These parameters are completely dependent on the AC sine wave on the anode and the DC gate voltage. The relationship model between the parameters (y1, y2, y3, y4, y5) and the applied AC sine wave and gate voltage can be established using a numerical polynomial control model for controlling and predicting the growth of ZnO NRs. The parameters (y1, y2, y3, y4, y5) are defined as outputs in the control model. The three inputs of the model were defined as follows: x1 – the peak-to-peak voltage of the AC sine wave on the anode; x2 – the frequency of the AC sine wave on the anode; x3 – the DC gate voltage. The analytical expression of the kth order multivariate polynomial can be written as follows: yj ¼
kab ka X k X X c¼0
j
pa;b;c xa1 xb2 xc3 ; j ¼ 1; 2; 3; 4; 5
ð3Þ
b¼0 a¼0
where j is the subscript of the five outputs (y1, y2, y3, y4, y5); x1, x2 and x3 are the inputs; a, b and c represent the degrees of inputs x1, x2 and x3, respectively; pja,b,c is the coefficient determined by fitting. Table 1 shows the values of x1, x2 and x3, which contains 24 different conditions. Using the simulation model, the outputs (y1, y2, y3, y4, y5) corresponding to the inputs (x1, x2, x3) listed in Table 1 were calculated in COMSOL. Afterwards, the relationship model between the outputs and the inputs were established using polynomial fitting based on the least square method. In order to evaluate the fitting accuracy of the polynomial model, the root-mean-square errors (RMSE) of polynomial model were analyzed. Fig. 5 shows the errors versus the polynomial order (from 1st to 4th). As we can see, the RMSE
Table 1
Values of x1, x2 and x3
x1 (Vpp)
x2 (MHz)
x3 (mV)
2.0/3.0/4.0/5.0
0.5/1
0/−40/−80
Fig. 5 The RMSE of the five outputs (y1, y2, y3, y4, y5) and average value versus the polynomial order.
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decreased dramatically from 1st to 2nd order. However, with a further increase in the polynomial order, the RMSE decreased slightly. Considering that a higher polynomial order will increase the computation complexity, we finally selected the 3rd order polynomial model as the numerical control model. The above analyses demonstrate that the magnitudes of y1 and y2 represent the vertical growth of the ZnO NRs on the ends of the anode and cathode, respectively. In addition, the magnitudes of the outputs y4 and y5 represent the lateral growth of the ZnO NRs on the ends of the anode and cathode, respectively. The increase in the magnitudes of y1, y2, y4 and y5 implies the electrode ends can provide more electrons. Under this condition, the reaction of NO3− ions with H2O can be greatly accelerated, and thus results in an increase in the OH− concentration and promotes the growth of the ZnO NRs. Output y3 characterizes the growth of the ZnO NRs on the electrode surface away from the ends. A negative value of y3 implies the supply of free electrons and the promotion of ZnO NRs growth, while a positive value of y3 implies the restraint of growth of the ZnO NRs. The increases in x1 and x3 will enhance the magnitude of all outputs, accelerate the reaction of NO3− ions with H2O and promote the growth of the ZnO NRs. However, due to the effect of surface capacitance, input x2 plays an opposite role. Specifically, decreasing the value of x2 will enhance the magnitude of all outputs, accelerate the reaction of NO3− ions with H2O and thus promote the growth of the ZnO NRs.
were located on the opposite ends of the anode and cathode. Fig. 2(k) and (l) show another example under the conditions of 3.8 Vpp, 1 MHz sine wave and −30 mV gate voltage. The outputs calculated using the model are −251.89 V m−1, −241.30 V m−1, 1.07 V m−1, −334.21 V m−1, 330.36 V m−1, which indicates a weaker growth of the NRs on the electrode ends compared with the conditions in Fig. 2(i) and ( j). These experiments further validated the effectiveness of prediction using the 3rd polynomial numerical control model. Fig. 6 shows the control of the morphology of the ZnO NRs growing on and between the two opposite ends of the cathode and anode by different gate voltage, AC sine-wave peak-to-peak value and frequency. In Fig. 6(a)–(c), the ZnO NRs were grown under the same sine-wave (3.6 Vpp, 1 MHz) with different gate voltage varying from −40 mV to GND and further to 40 mV, the average diameter of the ZnO NRs increased from 170 nm to
Validation of the prediction model Table 2 shows the output values calculated using the established 3rd polynomial model corresponding to the experimental growth conditions in Fig. 2(a)–(h). Furthermore, using the numerical control model, we calculated the outputs (y1, y2, y3, y4, y5) under the conditions of 4.5 Vpp, 1 MHz sine wave of anode with a −40 mV gate voltage applied. The calculated outputs (−277.58 V m−1, −260.73 V m−1, 20.64 V m−1, −385.32 V m−1, 382.97 V m−1) are also shown in Table 2, which indicates a strong growth of NRs on the electrode ends (due to the larger magnitudes of y1, y2, y3, y5), and an inhibitory effect on the NRs growth on electrode surface (due to a higher positive value of y3). Fig. 2(i) and ( j) show the SEM images of ZnO NRs grown under the conditions of 4.5 Vpp, 1 MHz sine wave of anode voltage and −40 mV gate voltage. The ZnO NRs grown
Fig. 6 SEM images of the morphologies of the ZnO NRs growing on and between the two opposite ends of the cathode and anode. The growth conditions are shown on the top of each image.
Table 2 The output values of the 3rd polynomial model and the average diameter and length of the ZnO NRs growing between the anode and cathode with different input values
x1 (Vpp)
x2 (MHz)
x3 (mV)
y1 (V m−1)
y2 (V m−1)
y3 (V m−1)
y4 (V m−1)
y5 (V m−1)
Ēxa (V m−1)
Db (nm)
Lc (μm)
3.6 3.6 3.6 3.6 4.5 3.8
1 1 1 0.5 1 1
40 0 −40 −40 −40 −30
−371.14 −288.59 −204.35 −332.32 −277.58 −251.89
−408.07 −302.43 −187.16 −283.55 −260.73 −241.30
−70.04 −5.65 19.80 5.24 20.64 1.07
−488.87 −392.36 −286.12 −455.05 −385.32 −334.21
526.40 411.95 282.53 402.69 382.97 330.36
507.64 402.16 284.33 428.87 384.15 332.29
530 460 170 480 430 360
2.7 2.2 0.65 2.4 2.1 1.7
Ēx is the average value of |y4| and |y5|. b D is the average diameter of the ZnO NRs laterally grown between the anode and cathode. c L is the average length of the ZnO NRs laterally grown between the anode and cathode.
a
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460 nm and further to 530 nm, the average length of NRs increased from 0.65 μm to 2.2 μm and further to 2.7 μm and the density were also gradually increased. Comparison of the growth results with different peak-to-peak value of the sinewave on the anode are shown in Fig. 6(a) and (d). With an increase of peak-to-peak value from 3.6 Vpp to 4.5 Vpp, the average diameter of the ZnO NRs increased from 170 nm to 430 nm, the average length increased from 0.65 μm to 2.1 μm and the density also increased. The growth morphology of NRs (360 nm, 1.7 μm) in Fig. 6(e) fell between Fig. 6(a) and (b), also between Fig. 6(a) and (d), due to the moderate gate voltage and anode sine-wave voltage. Fig. 6(a) and (f ) were conducted with the same gate voltage (−40 mV) and peak-to-peak value (3.6 Vpp) but different frequencies (1 MHz in (a) and 0.5 MHz
Fig. 7 (a) The relationship between Ēx (the average value of |y4| and |y5|) and D (the average diameter of the ZnO NRs). (b) the relationship between Ēx and L (the average length of the ZnO NRs). The values in brackets indicate the corresponding values of (x1, x2, x3).
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in (f )). Further comparison of Fig. 6(a) and (f ) shows that with a decrease of frequency, the average diameter of the ZnO NRs increased from 170 nm to 480 nm, the length increased from 0.65 μm to 2.4 μm and the density significantly increased. The results indicate that the gate voltage and the AC sine-wave on the anode could greatly dominate the diameter, length and density of ZnO NRs growing on and between the opposite ends of the two microelectrodes. As analysed above, the magnitudes of y1 and y2 determine the vertical growth of the ZnO NRs on the electrode ends, while the magnitudes of y4 and y5 determine the lateral growth of the ZnO NRs between the electrode ends. Considering the lateral growth of the ZnO NRs, the average value of |y4| and |y5| (represented by Ēx) and the average values of diameter (D) and length (L) of the NRs laterally grown between the anode and cathode are listed in Table 2. Fig. 7(a) and (b) reveal the positive correlation between Ēx (the average value of |y4| and |y5|) and D and L. As shown in Fig. 4 and Table 2, the magnitudes of y1 and y2 as well as the magnitudes of y4 and y5 are not strictly identical, which may result in slightly different formation of ZnO NRs on the anode and cathode. Taking Fig. 6(a) for example, the values of (y1, y2, y4, y5) are −204.35, −187.16, −286.12, 282.53, respectively and are listed in Table 2. The magnitude of y1 is larger than y2 and the magnitude of y4 is a little larger than y5. The results shown in Fig. 6(a) demonstrate that the ZnO NRs grew more densely on the left anode than the cathode, which is in accordance with the theoretical computations. The device chips with ZnO NRs grown were further characterized by properties such as electricity, photoelectricity, and thermoelectricity. The primary results are shown in Fig. 8. Fig. 8(a) shows the IV characteristics, which demonstrates the Schottky contacts between the ZnO NRs and Au electrodes. Fig. 8(b) shows the UV photoresponse of the device under a 365 nm LED (1.26 mW) in air at room temperature. The device shows the rise time (to 63%) of about 0.5 s and decay time (to 37%) of about 1 s with the current rising from 0.12 μA to 2.92 μA by 2333% with the applied bias of 3 V. Fig. 8(c) shows the negative temperature coefficient (NTC) characteristics of the ZnO NRs device. In virtue of a high surface-to-volume ratio by overhanging structure, the ZnO NRs device possesses an
Fig. 8 Various properties of the ZnO NRs chip. (a) I–V characteristics, (b) UV photoresponse under the illumination of a 365 nm UV LED with an intensity of 1.26 mW, (c) thermal sensitivity, RT = R0 exp(B/T ), where T is value of absolute temperature, RT is the resistance corresponding to the working temperature T, R0 = 4.67 Ω is the resistance when T → ∞ and B = 1926.4 K is the thermal constant.
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excellent heat exchange with ambient medium, which allows a good potential in ultra-fast thermal response and this work is in progress. All these experiments demonstrate the promising potentials for versatile sensor applications.
Conclusions In this study, an electric-field assisted hydrothermal growth method for integrating ZnO NRs with microstructures is proposed. By using this method, the growth position, direction and morphology of ZnO NRs can be easily controlled. A 3rd polynomial control model with the inputs ( peak-to-peak value and frequency of the AC sine wave on the anode, and gate voltage) and the outputs representing the growth position, direction and morphology of the ZnO NRs was established, which can be used for the prediction of the ZnO NRs growth. The electric field control methodology is of great importance for the control of ZnO NRs growth and makes it feasible to integrate ZnO NRs with microdevices. The sensing properties of the ZnO NRs demonstrate promising potential for versatile sensor applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) under grant no. 91123017 and the National Key Project of Scientific Instrument and Equipment Development under grant no. 2012YQ030261.
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