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Self-powered Broadband Photodetector using Plasmonic Titanium Nitride Amreen Ara Hussain, Bikash Sharma, Tapan Barman, and Arup Ratan Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00249 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 30, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Self-powered Broadband Photodetector using Plasmonic Titanium Nitride Amreen A. Hussain, Bikash Sharma, Tapan Barman, Arup R. Pal* Plasma Nanotech Lab, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Guwahati-781035, Assam, India *E-mail: [email protected], Phone: +91 361 2912073, Fax: +91 361 2273063
ABSTRACT: We report the demonstration of plasmonic titanium nitride (TiN) for fabrication of an efficient hybrid photodetector. A novel synthesis method based on plasma nanotechnology is utilized for producing air stable plasma polymerized aniline-TiN (PPATiN) nanocomposite and its integration in photodetector geometry. The device performs as a self-powered detector that responds to ultraviolet and visible light at zero bias. The photodetector has the advantage of broadband absorption and outcomes an enhanced photoresponse including high responsivity and detectivity under low light conditions. This work opens up a new direction for plasmonic TiN based hybrid nanocomposite and its exploitation in optoelectronic applications including imaging, light-wave communication and wire-free route for artificial vision. KEYWORDS: plasmonic, titanium nitride, broadband, self-powered, photodetector
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INTRODUCTION Nanoplasmonic applications and technologies have grown tremendously during the past decade1,2. Growing studies in the field of nanoplasmonics displayed that metal nanoparticles can be utilized to alter the electromagnetic field by trapping light from an external irradiance source to produce strong local field effects through the local surface plasmon resonance (LSPR). Traditional metal nanoparticles like gold (Au) and silver (Ag) are the extensively used plasmonic materials for practical applications including photodetectors3, spectroscopy4 and novel solar energy harvesting concepts5. Current approaches have remarkably addressed that plasmonic nanostructures; mainly Au and Ag can generate ‘hot carriers’ through nonradiative decay6. This novel concept of hot carrier generation provides a scheme for various applications such as water splitting7 and hydrogen (H2) dissociation8. These excitations also involve the direct conversion of light to electric current that can be realized in designing efficient photovoltaic devices or photodetectors9. In the latest search for alternative plasmonic material, Titanium Nitride (TiN) has sparked as a prospective source for broadband absorption covering the entire visible-infrared regions of the electromagnetic spectrum with additional advantages such as low loss, low cost, high thermal and oxidative stability, unlike Au and Ag10-13. Cortie et al. have shown the feasibility to design and fabricate plasmonically active structures using TiN by a combination of experiment and simulation10. It has been projected that practise of novel plasmonic materials like transition metal nitrides and conducting oxides can cover the whole spectral range of light absorption, and the idea of hot carrier generation enables the optimum utilization of the entire solar spectrum14. So far, the arena of plasmonic materials has delivered several exciting results from both experimental and theoretical point of view which has been considered as breakthroughs in their ground. However, transferring the recently identified plasmonic material like TiN into practical application is yet to be explored. Optoelectronic devices based on plasmonic 2 ACS Paragon Plus Environment
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nanostructures in contact with metals/semiconductors or organic small molecules, for example, TiO2, ZnO, MoS2 and graphene have been fabricated and investigated regarding plasmonic hot carrier generation, induced doping and possible phase transition15,16. In the context of plasmonic materials, we have recently addressed the synthesis of the composite structure by fusing gold nanoparticles into a p-type conducting polymer, polyaniline, which evolved as an efficient composite material for plasmonic photosensitization based on which an improved quality photodetector under photoconductive mode has been fabricated17. In recent years, it has been commonly projected that functional nano-systems, must be completely functional, and must not only have sensing, control and response capabilities but must be self-sufficient or self-driven as well. Currently, several self-powered photodetectors with high response speed have demonstrated that aims the possibility to operate independently, sustainably and wirelessly of the external power sources18-20. These photodetectors can work under zero-volt position or short-circuit conditions based on the photovoltaic behaviour of these devices, which are either driven by Schottky junctions or p-n heterojunctions without the need to consume external power21,22. This not only can enhance the adaptability of the device but also greatly reduces the size and weight of the system. However, the range of detectable light wavelengths is normally very narrow, and the limitations of the detectable light intensities are always high (~ 12.5-500 mW/cm2)22-26. It is, therefore, quite a challenge to fabricate simple nanostructured devices toward self-powered broadband detectors under low light conditions, which will be very promising for applications in upcoming nano-optoelectronic integrated circuits. Therefore, considering the requirement of self-powered photodetection and motivated by our previous reports17,27, and by the advantages of plasmonic TiN14 over other metal plasmonic nanostructures, an effort has been made to synthesize hybrid nanocomposite structures of plasma polymerized aniline (PPA) and plasmonic TiN. The PPA-TiN nanocomposite is 3 ACS Paragon Plus Environment
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prepared by a single step and dry route using plasma nanotechnology. The adopted processing technique has the merits that do not require high-temperature annealing and any lithography based assembly. Coupling the prepared PPA-TiN hybrid nanocomposite in optoelectronic device architecture, we report the first demonstration of plasmonic TiN based device: a hybrid self-powered photodetector with the advantage of broadband absorption and consistent photo-response including high responsivity and detectivity under low light conditions. EXPERIMENTAL SECTION Plasma setup and synthesis procedure The experiment has been done in a plasma reactor equipped with a water cooled planar magnetron fitted with a titanium target along with an RF electrode mounted just below the magnetron thus maintaining a gap of 10 cm28. Before deposition, the reactor is evacuated to a base pressure of 0.0026 Pa (2 × 10-5 Torr) using a turbo molecular pump backed by a primary dry pump. First, Nitrogen (N2) gas is introduced into the reactor at a partial pressure of 2.66 Pa (2 × 10-2 Torr). Subsequently, the monomer, Aniline vapour with Aniline flow of 25-30 sccm is injected into the reactor such that the final working pressure of the reactor is maintained at 6.66 Pa (5 × 10-2 Torr). Finally, the plasma polymerized aniline-titanium nitride (PPA-TiN) nanocomposite film is deposited by a combined process of pulsed DC reactive magnetron sputtering and RF plasma polymerization. The various plasma controlling parameters are summarized in Table S1. The details of material and electrical characterizations are provided in the supporting information. Device fabrication The hybrid device fabrication strategy can be divided into two steps. At first plasma deposition of PPA-TiN nanocomposite on top of an ITO coated glass (2.54 cm × 2.54 cm) is
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carried out using proper masking arrangement maintaining a thickness of 260 nm of the photoactive layer. After that, deposition of a 100 nm thick Aluminium (Al) electrode is done in a thermal evaporation chamber where high purity Al wires under high vacuum of 0.0026 Pa (2 × 10-5 Torr) are evaporated on the PPA-TiN/ITO structure. The fabricated hybrid photodetector has the configuration of Al/PPA-TiN/ITO with a lateral area of 6.5 × 10-5 cm2. RESULTS AND DISCUSSION Figure 1a shows FESEM image of PPA-TiN nanocomposite where a uniform and homogeneous film is deposited throughout the entire substrate. Figure 1b displays the EDX mapping of the nanocomposite that clearly reveals the elemental composition (Figure S1a) and distributed TiN nanoparticles (TiN NPs; green spots) in PPA-TiN nanocomposite. The C/N ratio derived from the EDX (Figure S1a) based on the atomic percent has value 4.5 which is quite deviated from the theoretical C/N atomic ratio of emeraldine base (EB) polyaniline which has a value of 6. A little rise of the nitrogen content in the nanocomposite is due to the presence of TiN in the film. Moreover, plasma polymerization also contributes to the deviated chemical composition in the nanocomposite29. TEM morphology of the synthesized TiN nanoparticles well embedded in the PPA matrix is presented in Figure 1c. The average size of TiN nanoparticles is 3.7 ± 0.5 nm (Figure S1b)). Figure 1d represents the HRTEM image and SAED pattern of TiN nanoparticles where the circular rings correspond to (111) and (200) crystallographic planes revealing their polycrystalline nature. The interplanar spacing is determined from the lattice fringes of PPA-TiN nanocomposite that are found to be 0.24 nm for TiN (111) and 0.21 nm for TiN (200) planes30. The structural evolution of the PPA-TiN nanocomposite is also examined from the XRD pattern shown in Figure S1c. The pattern has two nanocrystalline peaks that are indexed to (111) and (200) planes of fcc cubic TiN31-33 (JCPDS No. 00-002-1221). The lattice parameter determined from XRD is a = 0.42 nm. Consequently, the TiN (111) and (200) planes correspond to a 5 ACS Paragon Plus Environment
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lattice spacing (d) of 0.24 nm and 0.21 nm, respectively, which are consistent with the HRTEM results34. Further, the PPA-TiN nanocomposite is investigated for structural and optical properties. The FTIR spectra of TiN, PPA and PPA-TiN, is presented in Figure S2. The spectra of PPA and PPA-TiN (Figure S2b, c) present most of the characteristic peaks of the emeraldine base (EB) form of polyaniline with some additional new peaks, thus supporting the formation of polyaniline in the nanocomposite film. The characteristic wavenumber assignments of PPA and PPA-TiN are summarized in Table S2, and they are in good agreement with the literature reports35-38. The presence of aromatic groups is confirmed from the bands at 1602 cm-1 and 1498 cm-1 that are assigned to C=C stretch of quinoid (Q) and benzenoid (B) units respectively, corresponding to the two main units of the polymer backbone. Retention of the conjugated segments in PPA-TiN controls the assembly and enhances the long range ordering of hybrids owing to their tendency to π-stack. An indication of the branched and crosslinked structure of the PPA is found due to the presence of the bands at 696 cm-1 and 752 cm-1 that are assigned to 1,3 di-substituted aromatic ring and 1,2 di-substituted aromatic ring37. It is also observed that most of the absorption band positions of PPA-TiN nanocomposite film are shifted when compared with the spectrum of pure PPA film. This suggests the strong interaction between PPA and TiN associated with the interaction of titanium and nitrogen atom in PPA. Titanium is a transition metal, and it has a strong tendency to form coordination compound with the nitrogen atom in the polyaniline chain, but such interaction not only constrain the motion of polyaniline chains but also restricts the mode of vibration in polyaniline39,40. Thus, the strong interaction between titanium and nitrogen causes the shifting of bands along with the appearance of some new bands. Additionally, a band corresponding to nitrile vibration is observed at 2210 cm-1 associated to a possible chain termination. Finally, a sharp stretch at 1037 cm-1 implies the bonding of TiN in the polymer matrix thus 6 ACS Paragon Plus Environment
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confirming the formation of
nanocomposite41. Additionally, the GPC
PPA-TiN
chromatogram profile of PPA is shown in Figure S3a,b. The chromatogram shows a peak corresponding to a weight average molecular weight (Mw) of 33,322 g/mol with a polydispersity value of 1.152. The observed molecular weight is slightly smaller than some of the previously reported values for polyaniline (Mw = 40,000 ̶ 70,000 g/mol)42-44. This low molecular weight indicates that the polymer formed has branches and crosslinking, which prevents dissolving and therefore does not pass through the filtration process during sample preparation for GPC measurement44. This result is consistent with the FTIR spectrum which shows the prominent crosslinking of PPA. Figure 2, S4 shows the absorbance and normalized absorbance spectra of TiN, PPA and PPATiN nanocomposite. The absorption spectrum of PPA is significantly different from the spectrum of conventional polyaniline as none of the characteristic peak of emeraldine base (EB; 330, 630 nm), leucoemeraldine base (LB; 320-343 nm) and pernigraniline base (PNB; 283, 327 nm) are distinguishable, that provides the evidence of difference in the structure of PPA with that of conventinal polyaniline45-47. The position of the absorption band is dependent on the chain length as well as on the distribution of the benzenoid and quinoid units38. The absorption band of PPA is observed at 252 nm that clearly evidenced its short chain due to which the optical bandgap becomes higher (~ 4.9 eV), unlike the conventional polyaniline. Thus, PPA forms a unique polymeric structure. The spectrum of PPA-TiN nanocomposite is presented in Figure 2c that clearly illustrates two peaks: i.e. transitions at 265 nm, and 568 nm, respectively. The peak at 265 nm arises due to π → π* transition in PPA36,48 while the peak at 568 nm is due to LSPR in TiN12,49-51. An obvious red-shift of the plasmonic absorption peak with ∆λ = + 12 (Figure 2d, S4) is observed for the PPA-TiN nanocomposite as compared to TiN which might be due to the refractive index change of the environments possibly air versus the polymer matrix. 7 ACS Paragon Plus Environment
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Self-powered photodetectors are highly desired in various applications in optoelectronics including imaging, light-wave communications, the wire-free route for artificial vision etc52,53. In the field of light-wave optical systems, self-powered wireless photodetectors are extensively used in optical receivers and fibre optic communication52. Recently, self-powered photodetectors employing polymers, semiconductor nanorods/nanotubes/nanoparticles and quantum dots have become one of the prime focuses of current research that provide visual signals to a blind retina53. Self-powered photodetectors with broadband photo-detection also fascinate much attention owing to their functioning; independent of external power sources. In the present study, the displayed plasmonic absorption and well-organized material properties of as-synthesized PPA-TiN nanocomposite, direct its application towards optoelectronic devices. Thus, integrating the combined effect of plasmonic excitation in TiN and charge carrier transport through the conducting polymer (PPA), a self-powered hybrid photodetector is fabricated. Figure 3 provides an illustration of the device architecture where the PPA-TiN nanocomposite is sandwiched between two electrodes, bottom ITO and top Al. Figure 4a presents the current-voltage (I-V) characteristics of the hybrid photodetector under dark and white light (λ = 400-750 nm, λpeak ~ 570 nm) with an incident power density of 3.5 mW/cm2. The hybrid photodetector featured an excellent photovoltaic property with an open circuit voltage, VOC = 0.78 V and a short circuit current, ISC = 127.4 nA/cm2. A high VOC of the photodetector is the result of Schottky contact at ITO and PPA-TiN interface. At zero bias, the dark current is 1.82 nA/cm2. Therefore, the device can well function under selfpowered mode with a photosensitivity ((Ilight – Idark) / Idark) of ~ 70. The photo-response of the photodetector is tested under various irradiance intensities and is plotted in Figure 4b that showed gradually increasing photocurrent responses with increasing intensity. This observation is consistent with the fact that the amount of photo-generated charge carriers is proportional to the absorbed photon flux54. The dependence of photocurrent on irradiance 8 ACS Paragon Plus Environment
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intensity can be fitted by a power law: Ip =APθ where, Ip is the photocurrent, A is constant at a specific wavelength, P is the power density of the incident light, and θ is a parameter related to the trapping and recombination processes of the carriers in photodetectors. The best fitting shows θ = 0.2 (Figure 4c). Reports on a non-unity exponent with 0.5 < θ
Article
Self-powered Broadband Photodetector using Plasmonic Titanium Nitride Amreen Ara Hussain, Bikash Sharma, Tapan Barman, and Arup Ratan Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00249 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 30, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Self-powered Broadband Photodetector using Plasmonic Titanium Nitride Amreen A. Hussain, Bikash Sharma, Tapan Barman, Arup R. Pal* Plasma Nanotech Lab, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Guwahati-781035, Assam, India *E-mail: [email protected], Phone: +91 361 2912073, Fax: +91 361 2273063
ABSTRACT: We report the demonstration of plasmonic titanium nitride (TiN) for fabrication of an efficient hybrid photodetector. A novel synthesis method based on plasma nanotechnology is utilized for producing air stable plasma polymerized aniline-TiN (PPATiN) nanocomposite and its integration in photodetector geometry. The device performs as a self-powered detector that responds to ultraviolet and visible light at zero bias. The photodetector has the advantage of broadband absorption and outcomes an enhanced photoresponse including high responsivity and detectivity under low light conditions. This work opens up a new direction for plasmonic TiN based hybrid nanocomposite and its exploitation in optoelectronic applications including imaging, light-wave communication and wire-free route for artificial vision. KEYWORDS: plasmonic, titanium nitride, broadband, self-powered, photodetector
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INTRODUCTION Nanoplasmonic applications and technologies have grown tremendously during the past decade1,2. Growing studies in the field of nanoplasmonics displayed that metal nanoparticles can be utilized to alter the electromagnetic field by trapping light from an external irradiance source to produce strong local field effects through the local surface plasmon resonance (LSPR). Traditional metal nanoparticles like gold (Au) and silver (Ag) are the extensively used plasmonic materials for practical applications including photodetectors3, spectroscopy4 and novel solar energy harvesting concepts5. Current approaches have remarkably addressed that plasmonic nanostructures; mainly Au and Ag can generate ‘hot carriers’ through nonradiative decay6. This novel concept of hot carrier generation provides a scheme for various applications such as water splitting7 and hydrogen (H2) dissociation8. These excitations also involve the direct conversion of light to electric current that can be realized in designing efficient photovoltaic devices or photodetectors9. In the latest search for alternative plasmonic material, Titanium Nitride (TiN) has sparked as a prospective source for broadband absorption covering the entire visible-infrared regions of the electromagnetic spectrum with additional advantages such as low loss, low cost, high thermal and oxidative stability, unlike Au and Ag10-13. Cortie et al. have shown the feasibility to design and fabricate plasmonically active structures using TiN by a combination of experiment and simulation10. It has been projected that practise of novel plasmonic materials like transition metal nitrides and conducting oxides can cover the whole spectral range of light absorption, and the idea of hot carrier generation enables the optimum utilization of the entire solar spectrum14. So far, the arena of plasmonic materials has delivered several exciting results from both experimental and theoretical point of view which has been considered as breakthroughs in their ground. However, transferring the recently identified plasmonic material like TiN into practical application is yet to be explored. Optoelectronic devices based on plasmonic 2 ACS Paragon Plus Environment
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nanostructures in contact with metals/semiconductors or organic small molecules, for example, TiO2, ZnO, MoS2 and graphene have been fabricated and investigated regarding plasmonic hot carrier generation, induced doping and possible phase transition15,16. In the context of plasmonic materials, we have recently addressed the synthesis of the composite structure by fusing gold nanoparticles into a p-type conducting polymer, polyaniline, which evolved as an efficient composite material for plasmonic photosensitization based on which an improved quality photodetector under photoconductive mode has been fabricated17. In recent years, it has been commonly projected that functional nano-systems, must be completely functional, and must not only have sensing, control and response capabilities but must be self-sufficient or self-driven as well. Currently, several self-powered photodetectors with high response speed have demonstrated that aims the possibility to operate independently, sustainably and wirelessly of the external power sources18-20. These photodetectors can work under zero-volt position or short-circuit conditions based on the photovoltaic behaviour of these devices, which are either driven by Schottky junctions or p-n heterojunctions without the need to consume external power21,22. This not only can enhance the adaptability of the device but also greatly reduces the size and weight of the system. However, the range of detectable light wavelengths is normally very narrow, and the limitations of the detectable light intensities are always high (~ 12.5-500 mW/cm2)22-26. It is, therefore, quite a challenge to fabricate simple nanostructured devices toward self-powered broadband detectors under low light conditions, which will be very promising for applications in upcoming nano-optoelectronic integrated circuits. Therefore, considering the requirement of self-powered photodetection and motivated by our previous reports17,27, and by the advantages of plasmonic TiN14 over other metal plasmonic nanostructures, an effort has been made to synthesize hybrid nanocomposite structures of plasma polymerized aniline (PPA) and plasmonic TiN. The PPA-TiN nanocomposite is 3 ACS Paragon Plus Environment
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prepared by a single step and dry route using plasma nanotechnology. The adopted processing technique has the merits that do not require high-temperature annealing and any lithography based assembly. Coupling the prepared PPA-TiN hybrid nanocomposite in optoelectronic device architecture, we report the first demonstration of plasmonic TiN based device: a hybrid self-powered photodetector with the advantage of broadband absorption and consistent photo-response including high responsivity and detectivity under low light conditions. EXPERIMENTAL SECTION Plasma setup and synthesis procedure The experiment has been done in a plasma reactor equipped with a water cooled planar magnetron fitted with a titanium target along with an RF electrode mounted just below the magnetron thus maintaining a gap of 10 cm28. Before deposition, the reactor is evacuated to a base pressure of 0.0026 Pa (2 × 10-5 Torr) using a turbo molecular pump backed by a primary dry pump. First, Nitrogen (N2) gas is introduced into the reactor at a partial pressure of 2.66 Pa (2 × 10-2 Torr). Subsequently, the monomer, Aniline vapour with Aniline flow of 25-30 sccm is injected into the reactor such that the final working pressure of the reactor is maintained at 6.66 Pa (5 × 10-2 Torr). Finally, the plasma polymerized aniline-titanium nitride (PPA-TiN) nanocomposite film is deposited by a combined process of pulsed DC reactive magnetron sputtering and RF plasma polymerization. The various plasma controlling parameters are summarized in Table S1. The details of material and electrical characterizations are provided in the supporting information. Device fabrication The hybrid device fabrication strategy can be divided into two steps. At first plasma deposition of PPA-TiN nanocomposite on top of an ITO coated glass (2.54 cm × 2.54 cm) is
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carried out using proper masking arrangement maintaining a thickness of 260 nm of the photoactive layer. After that, deposition of a 100 nm thick Aluminium (Al) electrode is done in a thermal evaporation chamber where high purity Al wires under high vacuum of 0.0026 Pa (2 × 10-5 Torr) are evaporated on the PPA-TiN/ITO structure. The fabricated hybrid photodetector has the configuration of Al/PPA-TiN/ITO with a lateral area of 6.5 × 10-5 cm2. RESULTS AND DISCUSSION Figure 1a shows FESEM image of PPA-TiN nanocomposite where a uniform and homogeneous film is deposited throughout the entire substrate. Figure 1b displays the EDX mapping of the nanocomposite that clearly reveals the elemental composition (Figure S1a) and distributed TiN nanoparticles (TiN NPs; green spots) in PPA-TiN nanocomposite. The C/N ratio derived from the EDX (Figure S1a) based on the atomic percent has value 4.5 which is quite deviated from the theoretical C/N atomic ratio of emeraldine base (EB) polyaniline which has a value of 6. A little rise of the nitrogen content in the nanocomposite is due to the presence of TiN in the film. Moreover, plasma polymerization also contributes to the deviated chemical composition in the nanocomposite29. TEM morphology of the synthesized TiN nanoparticles well embedded in the PPA matrix is presented in Figure 1c. The average size of TiN nanoparticles is 3.7 ± 0.5 nm (Figure S1b)). Figure 1d represents the HRTEM image and SAED pattern of TiN nanoparticles where the circular rings correspond to (111) and (200) crystallographic planes revealing their polycrystalline nature. The interplanar spacing is determined from the lattice fringes of PPA-TiN nanocomposite that are found to be 0.24 nm for TiN (111) and 0.21 nm for TiN (200) planes30. The structural evolution of the PPA-TiN nanocomposite is also examined from the XRD pattern shown in Figure S1c. The pattern has two nanocrystalline peaks that are indexed to (111) and (200) planes of fcc cubic TiN31-33 (JCPDS No. 00-002-1221). The lattice parameter determined from XRD is a = 0.42 nm. Consequently, the TiN (111) and (200) planes correspond to a 5 ACS Paragon Plus Environment
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lattice spacing (d) of 0.24 nm and 0.21 nm, respectively, which are consistent with the HRTEM results34. Further, the PPA-TiN nanocomposite is investigated for structural and optical properties. The FTIR spectra of TiN, PPA and PPA-TiN, is presented in Figure S2. The spectra of PPA and PPA-TiN (Figure S2b, c) present most of the characteristic peaks of the emeraldine base (EB) form of polyaniline with some additional new peaks, thus supporting the formation of polyaniline in the nanocomposite film. The characteristic wavenumber assignments of PPA and PPA-TiN are summarized in Table S2, and they are in good agreement with the literature reports35-38. The presence of aromatic groups is confirmed from the bands at 1602 cm-1 and 1498 cm-1 that are assigned to C=C stretch of quinoid (Q) and benzenoid (B) units respectively, corresponding to the two main units of the polymer backbone. Retention of the conjugated segments in PPA-TiN controls the assembly and enhances the long range ordering of hybrids owing to their tendency to π-stack. An indication of the branched and crosslinked structure of the PPA is found due to the presence of the bands at 696 cm-1 and 752 cm-1 that are assigned to 1,3 di-substituted aromatic ring and 1,2 di-substituted aromatic ring37. It is also observed that most of the absorption band positions of PPA-TiN nanocomposite film are shifted when compared with the spectrum of pure PPA film. This suggests the strong interaction between PPA and TiN associated with the interaction of titanium and nitrogen atom in PPA. Titanium is a transition metal, and it has a strong tendency to form coordination compound with the nitrogen atom in the polyaniline chain, but such interaction not only constrain the motion of polyaniline chains but also restricts the mode of vibration in polyaniline39,40. Thus, the strong interaction between titanium and nitrogen causes the shifting of bands along with the appearance of some new bands. Additionally, a band corresponding to nitrile vibration is observed at 2210 cm-1 associated to a possible chain termination. Finally, a sharp stretch at 1037 cm-1 implies the bonding of TiN in the polymer matrix thus 6 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
confirming the formation of
nanocomposite41. Additionally, the GPC
PPA-TiN
chromatogram profile of PPA is shown in Figure S3a,b. The chromatogram shows a peak corresponding to a weight average molecular weight (Mw) of 33,322 g/mol with a polydispersity value of 1.152. The observed molecular weight is slightly smaller than some of the previously reported values for polyaniline (Mw = 40,000 ̶ 70,000 g/mol)42-44. This low molecular weight indicates that the polymer formed has branches and crosslinking, which prevents dissolving and therefore does not pass through the filtration process during sample preparation for GPC measurement44. This result is consistent with the FTIR spectrum which shows the prominent crosslinking of PPA. Figure 2, S4 shows the absorbance and normalized absorbance spectra of TiN, PPA and PPATiN nanocomposite. The absorption spectrum of PPA is significantly different from the spectrum of conventional polyaniline as none of the characteristic peak of emeraldine base (EB; 330, 630 nm), leucoemeraldine base (LB; 320-343 nm) and pernigraniline base (PNB; 283, 327 nm) are distinguishable, that provides the evidence of difference in the structure of PPA with that of conventinal polyaniline45-47. The position of the absorption band is dependent on the chain length as well as on the distribution of the benzenoid and quinoid units38. The absorption band of PPA is observed at 252 nm that clearly evidenced its short chain due to which the optical bandgap becomes higher (~ 4.9 eV), unlike the conventional polyaniline. Thus, PPA forms a unique polymeric structure. The spectrum of PPA-TiN nanocomposite is presented in Figure 2c that clearly illustrates two peaks: i.e. transitions at 265 nm, and 568 nm, respectively. The peak at 265 nm arises due to π → π* transition in PPA36,48 while the peak at 568 nm is due to LSPR in TiN12,49-51. An obvious red-shift of the plasmonic absorption peak with ∆λ = + 12 (Figure 2d, S4) is observed for the PPA-TiN nanocomposite as compared to TiN which might be due to the refractive index change of the environments possibly air versus the polymer matrix. 7 ACS Paragon Plus Environment
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Self-powered photodetectors are highly desired in various applications in optoelectronics including imaging, light-wave communications, the wire-free route for artificial vision etc52,53. In the field of light-wave optical systems, self-powered wireless photodetectors are extensively used in optical receivers and fibre optic communication52. Recently, self-powered photodetectors employing polymers, semiconductor nanorods/nanotubes/nanoparticles and quantum dots have become one of the prime focuses of current research that provide visual signals to a blind retina53. Self-powered photodetectors with broadband photo-detection also fascinate much attention owing to their functioning; independent of external power sources. In the present study, the displayed plasmonic absorption and well-organized material properties of as-synthesized PPA-TiN nanocomposite, direct its application towards optoelectronic devices. Thus, integrating the combined effect of plasmonic excitation in TiN and charge carrier transport through the conducting polymer (PPA), a self-powered hybrid photodetector is fabricated. Figure 3 provides an illustration of the device architecture where the PPA-TiN nanocomposite is sandwiched between two electrodes, bottom ITO and top Al. Figure 4a presents the current-voltage (I-V) characteristics of the hybrid photodetector under dark and white light (λ = 400-750 nm, λpeak ~ 570 nm) with an incident power density of 3.5 mW/cm2. The hybrid photodetector featured an excellent photovoltaic property with an open circuit voltage, VOC = 0.78 V and a short circuit current, ISC = 127.4 nA/cm2. A high VOC of the photodetector is the result of Schottky contact at ITO and PPA-TiN interface. At zero bias, the dark current is 1.82 nA/cm2. Therefore, the device can well function under selfpowered mode with a photosensitivity ((Ilight – Idark) / Idark) of ~ 70. The photo-response of the photodetector is tested under various irradiance intensities and is plotted in Figure 4b that showed gradually increasing photocurrent responses with increasing intensity. This observation is consistent with the fact that the amount of photo-generated charge carriers is proportional to the absorbed photon flux54. The dependence of photocurrent on irradiance 8 ACS Paragon Plus Environment
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intensity can be fitted by a power law: Ip =APθ where, Ip is the photocurrent, A is constant at a specific wavelength, P is the power density of the incident light, and θ is a parameter related to the trapping and recombination processes of the carriers in photodetectors. The best fitting shows θ = 0.2 (Figure 4c). Reports on a non-unity exponent with 0.5 < θ
