Letter pubs.acs.org/NanoLett
Catalyst-Directed Crystallographic Orientation Control of GaN Nanowire Growth Tevye R. Kuykendall,* M. Virginia P. Altoe, D. Frank Ogletree, and Shaul Aloni* Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: In this work, we demonstrate that catalyst composition can be used to direct the crystallographic growth axis of GaN nanowires. By adjusting the ratio of gold to nickel in a bimetallic catalyst, we achieved selective growth of dense, uniform nanowire arrays along two nonpolar directions. A gold-rich catalyst resulted in single-crystalline nanowire growth along the ⟨11̅00⟩ or m axis, whereas a nickel-rich catalyst resulted in nanowire growth along the ⟨1120̅ ⟩ or a axis. The same growth control was demonstrated on two different epitaxial substrates. Using proper conditions, many of the nanowires were observed to switch direction midgrowth, resulting in monolithic single-crystal structures with segments of two distinct orientations. Cathodoluminescence spectra revealed significant differences in the optical properties of these nanowire segments, which we attribute to the electronic structures of their semipolar {112̅2} or {11̅01} sidewalls. KEYWORDS: GaN, gallium nitride, growth direction, catalyst, nanowire, VLS, vapor−liquid−solid, gold, nickel, alloy toward finding a general mechanism for controlling the crystallographic direction of catalytic nanowire growth. Until now, nanowires have typically been grown using singlecomponent metal catalyst particles. However, looking at the role of metallic catalyst particles in conventional synthetic systems, we see that multicomponent alloys have been shown to improve the catalyst activity and selectivity relative to singlecomponent catalysts.11 This is often attributed to the dynamic behavior of the catalyst particle restructuring under varying reaction conditions.12 In this work, we demonstrate that a bimetallic catalyst can be used to control the crystallographic growth direction of the nanowire. By tuning the composition of a gold−nickel alloy, GaN nanowires were selectively grown along two distinct nonpolar axes. Although GaN can crystallize as one of two polymorphs, wurtzite or zincblende, the thermodynamically favorable wurtzite structure is almost exclusively present in nanowires. The growth axis of the nanowire and the surface energy anisotropy define the shape and surface termination of individual nanowires. The most commonly observed crystallographic growth directions for GaN nanowires are the nonpolar ⟨112̅0⟩13−17 and ⟨11̅00⟩13,16,18−20 and the polar ⟨0001⟩.18,21 Throughout this paper, these will be referred to as a-, m-, and caxis nanowires, respectively. Several higher-index directions have also been reported.13,22 Figure 1A shows a schematic of the hexagonal GaN structure indicating the crystallographic directions of a-, m-, and c-axis nanowire growth. Though
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emiconducting nanowires and nanotubes are often synthesized with the help of small metallic catalyst particles. Traditionally, catalysts have been employed in chemical synthesis to enhance reaction kinetics, select reaction products, and steer reaction pathways.1 Fifty years ago, Wagner and Ellis first demonstrated how metal catalyst particles induce nanowire (whisker) growth through the vapor−liquid−solid (VLS) mechanism.2 Here, gas-phase precursors adsorb on the surface and dissolve into the liquid catalyst droplet. Upon catalyst supersaturation, a nanowire nucleates and grows. The catalyst controls the growth kinetics by increasing the precursor sticking coefficient, providing a pathway for precursor decomposition at the gas−liquid interface, and confining the growth-front to the catalyst-nanowire interface, resulting in highly anisotropic 1D structures. Over the years, VLS, as well as the related vapor−solid−solid (VSS) and solution−liquid−solid (SLS) mechanisms, have been employed in the synthesis of a variety of elemental and compound-semiconductors using many different metal catalysts.3−5 Although early demonstrations of these mechanisms often produced arrays of nanowires resembling tangled noodles or wildfire-ravaged forests,6 refinements have given rise to highly ordered, well-defined arrays.7 Generally, these catalyst mediated growth mechanisms give rise to single-crystalline nanowires that grow along specific crystallographic directions. Often, there is a preferred growth axis, but occasionally, several orientations are observed. Changes in growth axis have been attributed to the nanowire diameter, choice of substrate, or change in the composition or partial pressure of the gas-phase precursors.8−10 Nonetheless, little progress has been made © XXXX American Chemical Society
Received: June 4, 2014 Revised: October 24, 2014
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Figure 1. Interplay of catalyst composition and epitaxy on the growth orientation of GaN nanowires. (A) Schematic of the hexagonal GaN structure, with arrows indicating the crystallographic directions of a-, m-, and c-axis nanowire growth. (B−E) SEM images of GaN nanowire arrays (500 nm scale bars). The top image in each frame was taken with the substrate tilted 60° from the surface normal. The lower image shows the substrate cross section. The insets illustrate the nanowire growth orientations with respect to the GaN crystal structure and substrate epitaxy. (B) m-axis GaN nanowires on r-plane α-Al2O3. (C) a-axis GaN nanowires on r-plane α-Al2O3. (D) m-axis GaN nanowires on (100) γ-LiAlO2. (E) a-axis GaN nanowires on (100) γ-LiAlO2. Panels F and G summarize the observed nanowire growth directions and illustrate how the nanowire orientation with respect to the substrate is defined by both the nanowire’s crystallographic growth axis and its epitaxial relationship. (Green lines represent GaN m planes.)
sequentially deposited, ultrathin (sub2 nm) films. The deposition method and analysis of the composition are discussed in Supporting Information SI-1. Selective growth along the a or m axis was achieved by tuning the gold−nickel ratio. Selective growth along the c axis was achieved by controlling the catalyst substrate interface (see Supporting Information SI-2). To confirm the role of the catalyst, and to distinguish it from effects due to the substrate, nanowires were grown on (11̅02) α-Al2O3, (100) γ-LiAlO2, and epitaxially grown c-plane (0001) GaN on (0001) α-Al2O3, due to their well-known epitaxial relations with GaN.25 Multiple substrates with different combinations of catalyst composition and thickness were grown simultaneously. The best results were achieved by choosing gold-rich or nickel-rich alloys with 1:50 III−V ratios in the temperature range of 780 to 820 °C. Figure 1B−E show scanning electron microscope (SEM) images of wire arrays grown with different substrates and catalyst compositions. The wires shown in Figure 1B and C were both grown on r-plane α-Al2O3 substrates. The gold-rich catalyzed wires maintain ±30° tilt from the substrate normal, whereas the nickel-rich catalyzed wires grow perpendicular to the substrate. Assuming that the GaN maintains the epitaxial relation of (112̅0) GaN plane parallel to the surface of the (11̅02) α-Al2O3 substrate,15,25,26 we can assign the growth axes to be m axis and a axis for gold-rich and nickel-rich catalysts, respectively. To confirm that the nanowire growth axis is controlled only by the catalyst composition and not the choice of substrate, we selected (100) γ-LiAlO2 as an alternative substrate. In this case the epitaxial relation is rotated by 30° along the c axis, with the (11̅00) GaN plane parallel to the (100) γ-LiAlO2 substrate surface. As expected, the gold-rich
nanowire growth along these three crystallographic directions has been frequently observed, there is little reported control over growth direction. Two notable exceptions18,23 employed substrate epitaxy to control the growth direction of GaN nanowires. As with thin films, nanowires can grow epitaxially on a substrate with proper lattice matching. In VLS growth, the catalyst particle confines the nanowire growth front at the catalyst−substrate interface. Kuykendall et al.18 showed using gold catalyst that, hexagonal cross section, c-axis nanowires grew perpendicular to (111) MgO, and triangular cross section, m-axis nanowires grew perpendicular to (100) γ-LiAlO2. The uniform arrays of c- and m-axis nanowires exhibited different optical properties.18,24 Here, we employ a different strategy to direct the crystallographic growth axis of GaN nanowires. We show that selective growth along the a and m axes can be achieved by tuning the composition of a gold−nickel bimetallic catalyst. To confirm that the catalyst plays the central role in determining the growth axis and to distinguish between the effects of the catalyst or substrate epitaxy, wire arrays are grown on several substrates with different epitaxial relationships. We compare the optical properties of the two growth axes by mapping the cathodoluminescence spectra of single wires in which the growth direction has been intentionally switched midgrowth. Dramatic differences between the two segments are revealed, which we attribute to changes in surface properties. The gallium nitride nanowires described here were grown by metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) and ammonia as the gallium and nitrogen sources, respectively (see Experimental Section). Gold−nickel alloys were used as the VLS catalyst. The catalyst composition was controlled by adjusting the thickness of B
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catalyzed wires are now perpendicular to the substrate (Figure 1D), consistent with m-axis growth, wheras the nickel-rich catalyzed wires are tilted ±30° to the substrate normal (Figure 1E), consistent with a-axis growth. These results suggest that the catalyst determines the growth axis of the nanowire. Wires grown with gold-rich catalyst are directed along the m axis, and those with nickel-rich catalyst are directed along the a axis. Figure 1F and G illustrate how the nanowire orientation with respect to the substrate is defined by both the nanowire crystallographic growth direction and the substrate epitaxial relationship. To confirm the epitaxial relationships and corresponding crystallographic nanowire growth directions inferred from the SEM images, we prepared cross-sectional samples for transmission electron microscopy (TEM). The GaN nanowire arrays were typically found embedded in a few-hundred nanometer thick epitaxial GaN film. The presence of similar film is commonly reported in nanowire literature and is often related to sterically frustrated nanowire growth or nonspecific film growth.26 In all cases, the expected epitaxial relationship was observed with no discontinuity of crystalline orientation between the nanowires and the film. The interfaces between GaN film and substrate were atomically sharp, and similar in quality to those reported in the thin-film literature. The epitaxial relationships are evident from both the high-resolution images and the nanobeam diffraction patterns shown in Figure 2. The TEM results confirm that the r-plane α-Al2O3 substrate surface is parallel to the GaN a plane, with (112̅0)GaN|| (11̅02)α-Al2O3 and [0001]GaN||[1̅101]α-Al2O3. The (100) γLiAlO2 substrate surface is parallel to the GaN m plane, with (11̅00)GaN||(100)γ-LiAlO2 and [0001]GaN||[010]γ-LiAlO2.27 The origin and nature of the underlying film are discussed in detail in Supporting Information SI-2. On both substrates, the nanowires are either perpendicular or at a 30° angle to the substrate normal as defined by the epitaxial relationship and the catalyst-defined growth axis. The nickel-rich catalyzed a-axis nanowires grow perpendicular to the r-plane sapphire surface, whereas the gold-rich catalyzed m-axis nanowires grow perpendicular to the (100) γ-LiAlO2 surface. The morphology and growth axes of individual nanowires were independently confirmed by TEM for many samples. In most cases, the wires were transferred to an electron transparent substrate by dry contact. Figure 3 shows TEM and Electron Diffraction (ED) data of m-axis wires (top) and aaxis wires (bottom) grown with gold- and nickel-rich catalyst, respectively. Low magnification images of m- and a-axis wires are shown in Figure 3A. The wires were tapered a few degrees, and the catalyst particle remains at the tip. The lattice resolved images in Figure 3B show that the wires are single-crystalline. The wires were typically found to be defect-free for diameters less than 200 nm. The ED patterns in Figure 3C were taken along the [0001] zone axis and indicate that the wires grow perpendicular to the(11̅00) and (112̅0) planes, along the m axis and a axis, respectively. To identify the side facets of the nanowires, 50−100 nm thick cross-sectional samples were prepared by microtome of nanowires embedded in epoxy resin. The ED patterns in Figure 3D were taken along the [11̅00] m axis and [112̅-20] a axis. The triangular cross sections seen in Figure 3D insets, show flat surfaces that are indexed as one polar {0001} face, and two symmetrical semipolar faces, indexed as {112̅2} for m-axis and {11̅01} for a-axis nanowires. The wire sidewalls, which are tilted by a few degrees toward the growth axis, are composed from {1122̅ } or {110̅ 1} terraces and
Figure 2. TEM cross sections showing the nanowire-substrate interface. Panels A−C show lattice resolved images of the interface (top left), the corresponding ED patterns (top right) and low magnification images bottom. Scale Bar: 400 nm. Color-coded circles are used in the ED patterns to indicate the GaN nanowires (red), αAl2O3 substrate (blue), and γ-LiAlO2 substrate (green). All of the images were taken along the GaN [0001] zone axis. (A) a-axis wires on r-plane α-Al2O3. (B) a-axis wires on (100) γ-LiAlO2. (C) m-axis wires on r-plane α-Al2O3. Note that the bending of the wire tips in panel A is due to capillary forces generated during curing of the epoxy used in cross section sample preparation. C
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Figure 3. TEM characterization of individual m-axis (top) and a-axis (bottom) GaN nanowires. (A) Low magnification image. (B) Lattice resolved image of GaN nanowires. (C) Electron diffraction patterns taken perpendicular to the growth axis, along the [0001] zone axis. The inset illustrations show the real space crystal structure and orientation of the nanowires with respect to the ED. (D) ED patterns of the nanowire cross sections taken with the zone axis along the nanowire growth direction. (E) Isometric space filling model of the nanowire. The c plane is facing left.
m- or a-plane steps, for a- and m-axis nanowires, respectively. Note that because TEM images are 2-dimentional projections of 3-dimentional objects, careful analysis of ED patterns at multiple orientations was essential for correct assignment of the nanowire growth axes. Further, care was taken to ensure that the ED patterns were free from rotation relative to the orientation of the wires. Space filling models are presented in Figure 3E to illustrate the subtle but important differences in surface structure of the m- and a-axis nanowires. To gain insight into the effect of catalyst composition on nanowire growth, a series of catalysts were prepared with increasing Ni/Au ratios. Although the catalyst composition was the most important factor in determining the growth direction, optimization of the temperature and partial pressures of H2/N2 carrier gases was necessary to obtain high yield and uniform nanowire coverage along the entire composition range. Figure 4A summarizes the morphology evolution as a function of Ni/ Au ratio (for more details see Supporting Information SI-3). The m-axis nanowires grew with good homogeneity for Ni concentrations between 14 and 80 at%. However, m-axis nanowire yield and uniformity were reduced at the higher growth temperature (820 C) with H2-rich carrier gas. The best a-axis nanowire growth was obtained for Ni concentrations between 86 and 91 at%, higher growth temperature, and H2rich carrier gas. Good yield and uniformity were obtained for a much smaller catalyst composition range, and the effects of carrier gas composition and growth temperature were more pronounced compared to m-axis growth. In contrast to previous work in which wires were prepared in a hot-wall reactor,13,18 in the cold-wall system used here, neither pure Au nor pure Ni were satisfactory catalysts. Pure Au did not result in wire growth, and pure Ni resulted in primarily ⟨112̅0⟩ growth with low yields.
We will now discuss the general role of the Ni/Au catalyst in GaN nanowire growth. First, establishing a well-defined liquid− solid interface is key for uniform VLS growth and high-yield nucleation. Kaplan’s work on the adhesion of liquid gold and nickel on sapphire gives us insight into this issue.28 The presence of nickel in the gold catalyst increases the catalyst adhesion to the ceramic or GaN substrate, whereas the presence of gold in the nickel catalyst lowers the dewetting temperature. This promotes the formation of the small catalyst droplets necessary for sustained nanowire growth. This can also explain the low yields under our growth conditions with pure Ni and Au films, where either catalyst adhesion or melting is compromised. Second, nickel segregation to the liquid alloy surface enhances ammonia decomposition, thus improving growth rate and yield.29 Third, the susceptibility of the nickel catalyst to poisoning by either carbon or oxygen containing species is consistent with the need for higher hydrogen partial pressures and higher growth temperatures to obtain good nanowire yields when using nickel-rich catalysts. Under these conditions any carbon or oxygen surface contaminants will react with hydrogen to form volatile compounds. The precise mechanism by which the catalyst composition directs the nanowire growth axis is difficult to pinpoint. Recent in situ TEM studies of elemental and III−V semiconductor nanowires show that the dominant growth mechanism is through catalyst supersaturation, step nucleation at the solid− liquid−gas interface, followed by rapid single-layer step growth across the liquid−solid interface.30 The catalyst composition may alter the liquid metal-nanowire interfacial energy at the growth front, stabilizing either the (1120̅ ) a or (110̅ 0) m planes, leading to a-axis wires with nickel-rich catalysts or maxis wires with gold-rich catalysts. Alternately, varying catalyst composition may change the relative step-nucleation barriers at the interfaces between the catalyst and GaN a or m planes, D
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Figure 4. Catalyst composition dependence on the growth direction and growth direction switching. (A) SEM images of nanowire arrays grown at optimal conditions as a function of the catalyst Ni concentration. (B) Low and high magnification TEM images and CBED diffraction pattern illustrating growth direction switching. The defocused ED pattern (right) confirms the wire switches from a-axis to m-axis. (C) Acceleration voltage dependent cathodoluminescence spectra integrated over the a-axis (red) and the m-axis (blue) part of the wire. (D) Monochromatic CL images of a GDS wire taken at 370, 410, and 510 nm showing the relative intensity distribution of light emitted from a nanowire excited with 2 kV electrons.
resulting in kinetically controlled growth. However, possible noncatalytic effects cannot be dismissed. For example, minute amounts of catalyst metals may be migrating or absorbed on the sides of the wires during growth, affecting the structure of the {1122̅ } and {110̅ 1} semipolar side faces. Although the estimated bulk-terminated surface energies for the two surfaces are similar,31 it is conceivable that migration of catalyst atoms, changes in the composition of the carrier gas, temperature, or other growth conditions will favor either of the semipolar surface terminations, and therefore the growth direction. Independent of the specific mechanism, the results of our study show that catalyst composition is the primary factor determining the direction of GaN nanowire growth. In the Supporting Information (SI-2), we contrast catalytic control of m- and a-axis growth with substrate control of a- and c-axis growth. Nanowire samples grown with an intermediate catalyst composition of 80−86 at% Ni show a mixture of a-axis and maxis wires, including some wires that show a sharp bend a few hundred nanometers away from the tip. The fraction of these wires increases with lower growth temperature and H2 partial pressure. High-resolution TEM imaging (Figure 4B) shows that these wires started to grow along the a axis and then changed direction to grow along the m axis. TEM images taken near the [0001] zone axis show a lack of defects or stacking faults, and indicate that the wires are single-crystalline through the region of the growth direction switching (GDS). We believe that the
GDS results from a transition from Ni-rich to Au-rich catalyst, either due to Ni incorporation into nanowire’s bulk or surface sites, or by formation of volatile Ni-containing species. GDS provides a unique opportunity to compare the properties of the two distinct m- and a-axis growth directions in a single, crystallographically monolithic nanowire. In contrast, comparisons between nanowires grown in separate runs or substrates always involve the possibility of artifacts related to changes in temperature, precursor contamination, or local environments. We use spatially resolved cathodoluminescence (CL) to investigate the optical properties of the nanowires. The top SEM image in Figure 4C shows a clear 30° bend, with the a-axis base oriented along the horizontal direction and the m-axis tip pointing up and to the right. The lower three images show monochromatic intensity maps collected at 370, 410, and 500 nm taken with a 2 kV electron beam and illustrate the striking differences between the a- and m-axis parts of the wire. The increased intensity at 370 nm from the m-axis part of the wire is clear. In Figure 4D we compare spectra as a function of the excitation depth from the a-axis (red) and m-axis (blue) segments of the nanowire shown in 4C. Cathodoluminescence maps were acquired at incident electron energies of 1, 2, and 5 kV, corresponding to maximum electron−hole pair generation depths of 12, 25, and 100 nm.32 The spectra show two distinct features: band edge luminescence (BEL) at 370 nm from transitions between the bottom of the conduction band and the top of the valence E
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band;33 and defect luminescence (DL), a broad peak centered at 500 nm associated with radiative recombination through electronic states in the middle of the band gap.33 At 5 kV the electron beam excites the entire volume of the nanowire. The aand m-axis sections show similar BEL and DL, with lower overall CL yield for the a-axis base despite its larger volume compared to the m-axis tip. As the excitation depth is reduced, a-axis BEL drops, and is completely absent at 1 kV. Typically DL is dominant at low excitation intensity. As excitation intensity increases, the defect recombination channel saturates, and BEL can be observed. The loss of BEL and the reduction in DL from the a-axis segment at low excitation voltage suggests an increase in nonradiative surface recombination. This is often attributed to midgap surface electronic states that pin the Fermi-level and bend bands, causing excited minority carriers (holes in the case of n-type nanowires) to drift toward the surface, where they can recombine nonradiatively.34,35 At higher excitation voltage, the relative importance of surface recombination is reduced, and the bulk contributions of the aand m-axis segments are similar. The brighter, {1122̅ } surface of the m-axis wires shows a lower surface recombination rate or reduced band-bending relative to the {11̅01} surface of the aaxis wires. In summary, we have shown that catalyst composition can control the crystallographic growth axis of GaN nanowires. Bimetallic catalysts give enhanced control over MOCVD growth, leading to improved yield and uniformity of GaN nanowire arrays. Moreover, dynamic changes in catalyst composition during growth can cause growth direction switching. Finally, we were able to link differences in optical properties of the nanowires to changes in the crystallographic surface orientation, which are determined by the growth axis of the nanowire.
Andor Newton camera. Monochromatic images were acquired with spectrometer set as a 20 nm bandpass filter using a Hamatsu H7195 photomultiplier assembly. All materials and methods described here are available through the user program at the Molecular Foundry.
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ASSOCIATED CONTENT
S Supporting Information *
1. Gold−nickel catalyst preparation and characterization. 2. Steric frustration and GaN film growth. 3. GaN nanowires grown with gold−nickel catalyst This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was performed at the Molecular Foundry and its National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, with support from the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Contract DE-AC02-05CH1123. The authors would like to thank Marissa Libbee for assisting with the preparation of crossectional TEM samples.
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EXPERIMENTAL SECTION GaN nanowire growth was carried out in a Thomas Swan showerhead 3 × 2 MOCVD cold-wall reactor. The growth substrates were prepared by thermal evaporation of gold and nickel on r-plane and c-plane α-Al2O3 wafers purchased from Monocrystal and (100) γ-LiAlO2 substrates from MTI Corporation. Hydrogen at 30 sccm was bubbled through TMG and diluted with 20 sccm hydrogen. The TMG bubbler was kept at 5 °C, and the reactor pressure was set to 140 mbar. Ammonia at 500 sccm was used, with an additional 450 sccm of hydrogen or nitrogen as the carrier gas. Growth temperature was 780° or 820 °C. A range of catalyst compositions and thicknesses were used. In general, m-axis GaN nanowires were obtained for a fairly wide range of compositions: from 14 to 86 at% nickel and a total thickness of 0.4−1.3 nm using higher nitrogen partial pressures. a-axis wires were obtained for a narrow range of compositions: from 92 to 96 at% nickel and a total thickness of 0.4−0.8 nm at 820 °C using higher hydrogen partial pressures. The TEM characterization has been carried out on a JEOL 2100F electron microscope operated at 200 kV equipped with Oxford INCA energy dispersive electron X-ray spectrometer and Tridiem Gatan imaging filter and spectrometer. Crossectional polishing and ion mill at facilities at NCEM were used for cross-sectional sample preparation. CL measurements were performed using a Zeiss Supra 55 VP-FESEM integrated with home-built, fiber-based light collection system. The spectra were acquired using a SP2300i Acton spectrometer (Princeton Instruments) equipped with
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Catalyst-Directed Crystallographic Orientation Control of GaN Nanowire Growth Tevye R. Kuykendall,* M. Virginia P. Altoe, D. Frank Ogletree, and Shaul Aloni* Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: In this work, we demonstrate that catalyst composition can be used to direct the crystallographic growth axis of GaN nanowires. By adjusting the ratio of gold to nickel in a bimetallic catalyst, we achieved selective growth of dense, uniform nanowire arrays along two nonpolar directions. A gold-rich catalyst resulted in single-crystalline nanowire growth along the ⟨11̅00⟩ or m axis, whereas a nickel-rich catalyst resulted in nanowire growth along the ⟨1120̅ ⟩ or a axis. The same growth control was demonstrated on two different epitaxial substrates. Using proper conditions, many of the nanowires were observed to switch direction midgrowth, resulting in monolithic single-crystal structures with segments of two distinct orientations. Cathodoluminescence spectra revealed significant differences in the optical properties of these nanowire segments, which we attribute to the electronic structures of their semipolar {112̅2} or {11̅01} sidewalls. KEYWORDS: GaN, gallium nitride, growth direction, catalyst, nanowire, VLS, vapor−liquid−solid, gold, nickel, alloy toward finding a general mechanism for controlling the crystallographic direction of catalytic nanowire growth. Until now, nanowires have typically been grown using singlecomponent metal catalyst particles. However, looking at the role of metallic catalyst particles in conventional synthetic systems, we see that multicomponent alloys have been shown to improve the catalyst activity and selectivity relative to singlecomponent catalysts.11 This is often attributed to the dynamic behavior of the catalyst particle restructuring under varying reaction conditions.12 In this work, we demonstrate that a bimetallic catalyst can be used to control the crystallographic growth direction of the nanowire. By tuning the composition of a gold−nickel alloy, GaN nanowires were selectively grown along two distinct nonpolar axes. Although GaN can crystallize as one of two polymorphs, wurtzite or zincblende, the thermodynamically favorable wurtzite structure is almost exclusively present in nanowires. The growth axis of the nanowire and the surface energy anisotropy define the shape and surface termination of individual nanowires. The most commonly observed crystallographic growth directions for GaN nanowires are the nonpolar ⟨112̅0⟩13−17 and ⟨11̅00⟩13,16,18−20 and the polar ⟨0001⟩.18,21 Throughout this paper, these will be referred to as a-, m-, and caxis nanowires, respectively. Several higher-index directions have also been reported.13,22 Figure 1A shows a schematic of the hexagonal GaN structure indicating the crystallographic directions of a-, m-, and c-axis nanowire growth. Though
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emiconducting nanowires and nanotubes are often synthesized with the help of small metallic catalyst particles. Traditionally, catalysts have been employed in chemical synthesis to enhance reaction kinetics, select reaction products, and steer reaction pathways.1 Fifty years ago, Wagner and Ellis first demonstrated how metal catalyst particles induce nanowire (whisker) growth through the vapor−liquid−solid (VLS) mechanism.2 Here, gas-phase precursors adsorb on the surface and dissolve into the liquid catalyst droplet. Upon catalyst supersaturation, a nanowire nucleates and grows. The catalyst controls the growth kinetics by increasing the precursor sticking coefficient, providing a pathway for precursor decomposition at the gas−liquid interface, and confining the growth-front to the catalyst-nanowire interface, resulting in highly anisotropic 1D structures. Over the years, VLS, as well as the related vapor−solid−solid (VSS) and solution−liquid−solid (SLS) mechanisms, have been employed in the synthesis of a variety of elemental and compound-semiconductors using many different metal catalysts.3−5 Although early demonstrations of these mechanisms often produced arrays of nanowires resembling tangled noodles or wildfire-ravaged forests,6 refinements have given rise to highly ordered, well-defined arrays.7 Generally, these catalyst mediated growth mechanisms give rise to single-crystalline nanowires that grow along specific crystallographic directions. Often, there is a preferred growth axis, but occasionally, several orientations are observed. Changes in growth axis have been attributed to the nanowire diameter, choice of substrate, or change in the composition or partial pressure of the gas-phase precursors.8−10 Nonetheless, little progress has been made © XXXX American Chemical Society
Received: June 4, 2014 Revised: October 24, 2014
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Figure 1. Interplay of catalyst composition and epitaxy on the growth orientation of GaN nanowires. (A) Schematic of the hexagonal GaN structure, with arrows indicating the crystallographic directions of a-, m-, and c-axis nanowire growth. (B−E) SEM images of GaN nanowire arrays (500 nm scale bars). The top image in each frame was taken with the substrate tilted 60° from the surface normal. The lower image shows the substrate cross section. The insets illustrate the nanowire growth orientations with respect to the GaN crystal structure and substrate epitaxy. (B) m-axis GaN nanowires on r-plane α-Al2O3. (C) a-axis GaN nanowires on r-plane α-Al2O3. (D) m-axis GaN nanowires on (100) γ-LiAlO2. (E) a-axis GaN nanowires on (100) γ-LiAlO2. Panels F and G summarize the observed nanowire growth directions and illustrate how the nanowire orientation with respect to the substrate is defined by both the nanowire’s crystallographic growth axis and its epitaxial relationship. (Green lines represent GaN m planes.)
sequentially deposited, ultrathin (sub2 nm) films. The deposition method and analysis of the composition are discussed in Supporting Information SI-1. Selective growth along the a or m axis was achieved by tuning the gold−nickel ratio. Selective growth along the c axis was achieved by controlling the catalyst substrate interface (see Supporting Information SI-2). To confirm the role of the catalyst, and to distinguish it from effects due to the substrate, nanowires were grown on (11̅02) α-Al2O3, (100) γ-LiAlO2, and epitaxially grown c-plane (0001) GaN on (0001) α-Al2O3, due to their well-known epitaxial relations with GaN.25 Multiple substrates with different combinations of catalyst composition and thickness were grown simultaneously. The best results were achieved by choosing gold-rich or nickel-rich alloys with 1:50 III−V ratios in the temperature range of 780 to 820 °C. Figure 1B−E show scanning electron microscope (SEM) images of wire arrays grown with different substrates and catalyst compositions. The wires shown in Figure 1B and C were both grown on r-plane α-Al2O3 substrates. The gold-rich catalyzed wires maintain ±30° tilt from the substrate normal, whereas the nickel-rich catalyzed wires grow perpendicular to the substrate. Assuming that the GaN maintains the epitaxial relation of (112̅0) GaN plane parallel to the surface of the (11̅02) α-Al2O3 substrate,15,25,26 we can assign the growth axes to be m axis and a axis for gold-rich and nickel-rich catalysts, respectively. To confirm that the nanowire growth axis is controlled only by the catalyst composition and not the choice of substrate, we selected (100) γ-LiAlO2 as an alternative substrate. In this case the epitaxial relation is rotated by 30° along the c axis, with the (11̅00) GaN plane parallel to the (100) γ-LiAlO2 substrate surface. As expected, the gold-rich
nanowire growth along these three crystallographic directions has been frequently observed, there is little reported control over growth direction. Two notable exceptions18,23 employed substrate epitaxy to control the growth direction of GaN nanowires. As with thin films, nanowires can grow epitaxially on a substrate with proper lattice matching. In VLS growth, the catalyst particle confines the nanowire growth front at the catalyst−substrate interface. Kuykendall et al.18 showed using gold catalyst that, hexagonal cross section, c-axis nanowires grew perpendicular to (111) MgO, and triangular cross section, m-axis nanowires grew perpendicular to (100) γ-LiAlO2. The uniform arrays of c- and m-axis nanowires exhibited different optical properties.18,24 Here, we employ a different strategy to direct the crystallographic growth axis of GaN nanowires. We show that selective growth along the a and m axes can be achieved by tuning the composition of a gold−nickel bimetallic catalyst. To confirm that the catalyst plays the central role in determining the growth axis and to distinguish between the effects of the catalyst or substrate epitaxy, wire arrays are grown on several substrates with different epitaxial relationships. We compare the optical properties of the two growth axes by mapping the cathodoluminescence spectra of single wires in which the growth direction has been intentionally switched midgrowth. Dramatic differences between the two segments are revealed, which we attribute to changes in surface properties. The gallium nitride nanowires described here were grown by metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) and ammonia as the gallium and nitrogen sources, respectively (see Experimental Section). Gold−nickel alloys were used as the VLS catalyst. The catalyst composition was controlled by adjusting the thickness of B
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catalyzed wires are now perpendicular to the substrate (Figure 1D), consistent with m-axis growth, wheras the nickel-rich catalyzed wires are tilted ±30° to the substrate normal (Figure 1E), consistent with a-axis growth. These results suggest that the catalyst determines the growth axis of the nanowire. Wires grown with gold-rich catalyst are directed along the m axis, and those with nickel-rich catalyst are directed along the a axis. Figure 1F and G illustrate how the nanowire orientation with respect to the substrate is defined by both the nanowire crystallographic growth direction and the substrate epitaxial relationship. To confirm the epitaxial relationships and corresponding crystallographic nanowire growth directions inferred from the SEM images, we prepared cross-sectional samples for transmission electron microscopy (TEM). The GaN nanowire arrays were typically found embedded in a few-hundred nanometer thick epitaxial GaN film. The presence of similar film is commonly reported in nanowire literature and is often related to sterically frustrated nanowire growth or nonspecific film growth.26 In all cases, the expected epitaxial relationship was observed with no discontinuity of crystalline orientation between the nanowires and the film. The interfaces between GaN film and substrate were atomically sharp, and similar in quality to those reported in the thin-film literature. The epitaxial relationships are evident from both the high-resolution images and the nanobeam diffraction patterns shown in Figure 2. The TEM results confirm that the r-plane α-Al2O3 substrate surface is parallel to the GaN a plane, with (112̅0)GaN|| (11̅02)α-Al2O3 and [0001]GaN||[1̅101]α-Al2O3. The (100) γLiAlO2 substrate surface is parallel to the GaN m plane, with (11̅00)GaN||(100)γ-LiAlO2 and [0001]GaN||[010]γ-LiAlO2.27 The origin and nature of the underlying film are discussed in detail in Supporting Information SI-2. On both substrates, the nanowires are either perpendicular or at a 30° angle to the substrate normal as defined by the epitaxial relationship and the catalyst-defined growth axis. The nickel-rich catalyzed a-axis nanowires grow perpendicular to the r-plane sapphire surface, whereas the gold-rich catalyzed m-axis nanowires grow perpendicular to the (100) γ-LiAlO2 surface. The morphology and growth axes of individual nanowires were independently confirmed by TEM for many samples. In most cases, the wires were transferred to an electron transparent substrate by dry contact. Figure 3 shows TEM and Electron Diffraction (ED) data of m-axis wires (top) and aaxis wires (bottom) grown with gold- and nickel-rich catalyst, respectively. Low magnification images of m- and a-axis wires are shown in Figure 3A. The wires were tapered a few degrees, and the catalyst particle remains at the tip. The lattice resolved images in Figure 3B show that the wires are single-crystalline. The wires were typically found to be defect-free for diameters less than 200 nm. The ED patterns in Figure 3C were taken along the [0001] zone axis and indicate that the wires grow perpendicular to the(11̅00) and (112̅0) planes, along the m axis and a axis, respectively. To identify the side facets of the nanowires, 50−100 nm thick cross-sectional samples were prepared by microtome of nanowires embedded in epoxy resin. The ED patterns in Figure 3D were taken along the [11̅00] m axis and [112̅-20] a axis. The triangular cross sections seen in Figure 3D insets, show flat surfaces that are indexed as one polar {0001} face, and two symmetrical semipolar faces, indexed as {112̅2} for m-axis and {11̅01} for a-axis nanowires. The wire sidewalls, which are tilted by a few degrees toward the growth axis, are composed from {1122̅ } or {110̅ 1} terraces and
Figure 2. TEM cross sections showing the nanowire-substrate interface. Panels A−C show lattice resolved images of the interface (top left), the corresponding ED patterns (top right) and low magnification images bottom. Scale Bar: 400 nm. Color-coded circles are used in the ED patterns to indicate the GaN nanowires (red), αAl2O3 substrate (blue), and γ-LiAlO2 substrate (green). All of the images were taken along the GaN [0001] zone axis. (A) a-axis wires on r-plane α-Al2O3. (B) a-axis wires on (100) γ-LiAlO2. (C) m-axis wires on r-plane α-Al2O3. Note that the bending of the wire tips in panel A is due to capillary forces generated during curing of the epoxy used in cross section sample preparation. C
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Figure 3. TEM characterization of individual m-axis (top) and a-axis (bottom) GaN nanowires. (A) Low magnification image. (B) Lattice resolved image of GaN nanowires. (C) Electron diffraction patterns taken perpendicular to the growth axis, along the [0001] zone axis. The inset illustrations show the real space crystal structure and orientation of the nanowires with respect to the ED. (D) ED patterns of the nanowire cross sections taken with the zone axis along the nanowire growth direction. (E) Isometric space filling model of the nanowire. The c plane is facing left.
m- or a-plane steps, for a- and m-axis nanowires, respectively. Note that because TEM images are 2-dimentional projections of 3-dimentional objects, careful analysis of ED patterns at multiple orientations was essential for correct assignment of the nanowire growth axes. Further, care was taken to ensure that the ED patterns were free from rotation relative to the orientation of the wires. Space filling models are presented in Figure 3E to illustrate the subtle but important differences in surface structure of the m- and a-axis nanowires. To gain insight into the effect of catalyst composition on nanowire growth, a series of catalysts were prepared with increasing Ni/Au ratios. Although the catalyst composition was the most important factor in determining the growth direction, optimization of the temperature and partial pressures of H2/N2 carrier gases was necessary to obtain high yield and uniform nanowire coverage along the entire composition range. Figure 4A summarizes the morphology evolution as a function of Ni/ Au ratio (for more details see Supporting Information SI-3). The m-axis nanowires grew with good homogeneity for Ni concentrations between 14 and 80 at%. However, m-axis nanowire yield and uniformity were reduced at the higher growth temperature (820 C) with H2-rich carrier gas. The best a-axis nanowire growth was obtained for Ni concentrations between 86 and 91 at%, higher growth temperature, and H2rich carrier gas. Good yield and uniformity were obtained for a much smaller catalyst composition range, and the effects of carrier gas composition and growth temperature were more pronounced compared to m-axis growth. In contrast to previous work in which wires were prepared in a hot-wall reactor,13,18 in the cold-wall system used here, neither pure Au nor pure Ni were satisfactory catalysts. Pure Au did not result in wire growth, and pure Ni resulted in primarily ⟨112̅0⟩ growth with low yields.
We will now discuss the general role of the Ni/Au catalyst in GaN nanowire growth. First, establishing a well-defined liquid− solid interface is key for uniform VLS growth and high-yield nucleation. Kaplan’s work on the adhesion of liquid gold and nickel on sapphire gives us insight into this issue.28 The presence of nickel in the gold catalyst increases the catalyst adhesion to the ceramic or GaN substrate, whereas the presence of gold in the nickel catalyst lowers the dewetting temperature. This promotes the formation of the small catalyst droplets necessary for sustained nanowire growth. This can also explain the low yields under our growth conditions with pure Ni and Au films, where either catalyst adhesion or melting is compromised. Second, nickel segregation to the liquid alloy surface enhances ammonia decomposition, thus improving growth rate and yield.29 Third, the susceptibility of the nickel catalyst to poisoning by either carbon or oxygen containing species is consistent with the need for higher hydrogen partial pressures and higher growth temperatures to obtain good nanowire yields when using nickel-rich catalysts. Under these conditions any carbon or oxygen surface contaminants will react with hydrogen to form volatile compounds. The precise mechanism by which the catalyst composition directs the nanowire growth axis is difficult to pinpoint. Recent in situ TEM studies of elemental and III−V semiconductor nanowires show that the dominant growth mechanism is through catalyst supersaturation, step nucleation at the solid− liquid−gas interface, followed by rapid single-layer step growth across the liquid−solid interface.30 The catalyst composition may alter the liquid metal-nanowire interfacial energy at the growth front, stabilizing either the (1120̅ ) a or (110̅ 0) m planes, leading to a-axis wires with nickel-rich catalysts or maxis wires with gold-rich catalysts. Alternately, varying catalyst composition may change the relative step-nucleation barriers at the interfaces between the catalyst and GaN a or m planes, D
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Figure 4. Catalyst composition dependence on the growth direction and growth direction switching. (A) SEM images of nanowire arrays grown at optimal conditions as a function of the catalyst Ni concentration. (B) Low and high magnification TEM images and CBED diffraction pattern illustrating growth direction switching. The defocused ED pattern (right) confirms the wire switches from a-axis to m-axis. (C) Acceleration voltage dependent cathodoluminescence spectra integrated over the a-axis (red) and the m-axis (blue) part of the wire. (D) Monochromatic CL images of a GDS wire taken at 370, 410, and 510 nm showing the relative intensity distribution of light emitted from a nanowire excited with 2 kV electrons.
resulting in kinetically controlled growth. However, possible noncatalytic effects cannot be dismissed. For example, minute amounts of catalyst metals may be migrating or absorbed on the sides of the wires during growth, affecting the structure of the {1122̅ } and {110̅ 1} semipolar side faces. Although the estimated bulk-terminated surface energies for the two surfaces are similar,31 it is conceivable that migration of catalyst atoms, changes in the composition of the carrier gas, temperature, or other growth conditions will favor either of the semipolar surface terminations, and therefore the growth direction. Independent of the specific mechanism, the results of our study show that catalyst composition is the primary factor determining the direction of GaN nanowire growth. In the Supporting Information (SI-2), we contrast catalytic control of m- and a-axis growth with substrate control of a- and c-axis growth. Nanowire samples grown with an intermediate catalyst composition of 80−86 at% Ni show a mixture of a-axis and maxis wires, including some wires that show a sharp bend a few hundred nanometers away from the tip. The fraction of these wires increases with lower growth temperature and H2 partial pressure. High-resolution TEM imaging (Figure 4B) shows that these wires started to grow along the a axis and then changed direction to grow along the m axis. TEM images taken near the [0001] zone axis show a lack of defects or stacking faults, and indicate that the wires are single-crystalline through the region of the growth direction switching (GDS). We believe that the
GDS results from a transition from Ni-rich to Au-rich catalyst, either due to Ni incorporation into nanowire’s bulk or surface sites, or by formation of volatile Ni-containing species. GDS provides a unique opportunity to compare the properties of the two distinct m- and a-axis growth directions in a single, crystallographically monolithic nanowire. In contrast, comparisons between nanowires grown in separate runs or substrates always involve the possibility of artifacts related to changes in temperature, precursor contamination, or local environments. We use spatially resolved cathodoluminescence (CL) to investigate the optical properties of the nanowires. The top SEM image in Figure 4C shows a clear 30° bend, with the a-axis base oriented along the horizontal direction and the m-axis tip pointing up and to the right. The lower three images show monochromatic intensity maps collected at 370, 410, and 500 nm taken with a 2 kV electron beam and illustrate the striking differences between the a- and m-axis parts of the wire. The increased intensity at 370 nm from the m-axis part of the wire is clear. In Figure 4D we compare spectra as a function of the excitation depth from the a-axis (red) and m-axis (blue) segments of the nanowire shown in 4C. Cathodoluminescence maps were acquired at incident electron energies of 1, 2, and 5 kV, corresponding to maximum electron−hole pair generation depths of 12, 25, and 100 nm.32 The spectra show two distinct features: band edge luminescence (BEL) at 370 nm from transitions between the bottom of the conduction band and the top of the valence E
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band;33 and defect luminescence (DL), a broad peak centered at 500 nm associated with radiative recombination through electronic states in the middle of the band gap.33 At 5 kV the electron beam excites the entire volume of the nanowire. The aand m-axis sections show similar BEL and DL, with lower overall CL yield for the a-axis base despite its larger volume compared to the m-axis tip. As the excitation depth is reduced, a-axis BEL drops, and is completely absent at 1 kV. Typically DL is dominant at low excitation intensity. As excitation intensity increases, the defect recombination channel saturates, and BEL can be observed. The loss of BEL and the reduction in DL from the a-axis segment at low excitation voltage suggests an increase in nonradiative surface recombination. This is often attributed to midgap surface electronic states that pin the Fermi-level and bend bands, causing excited minority carriers (holes in the case of n-type nanowires) to drift toward the surface, where they can recombine nonradiatively.34,35 At higher excitation voltage, the relative importance of surface recombination is reduced, and the bulk contributions of the aand m-axis segments are similar. The brighter, {1122̅ } surface of the m-axis wires shows a lower surface recombination rate or reduced band-bending relative to the {11̅01} surface of the aaxis wires. In summary, we have shown that catalyst composition can control the crystallographic growth axis of GaN nanowires. Bimetallic catalysts give enhanced control over MOCVD growth, leading to improved yield and uniformity of GaN nanowire arrays. Moreover, dynamic changes in catalyst composition during growth can cause growth direction switching. Finally, we were able to link differences in optical properties of the nanowires to changes in the crystallographic surface orientation, which are determined by the growth axis of the nanowire.
Andor Newton camera. Monochromatic images were acquired with spectrometer set as a 20 nm bandpass filter using a Hamatsu H7195 photomultiplier assembly. All materials and methods described here are available through the user program at the Molecular Foundry.
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ASSOCIATED CONTENT
S Supporting Information *
1. Gold−nickel catalyst preparation and characterization. 2. Steric frustration and GaN film growth. 3. GaN nanowires grown with gold−nickel catalyst This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was performed at the Molecular Foundry and its National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, with support from the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Contract DE-AC02-05CH1123. The authors would like to thank Marissa Libbee for assisting with the preparation of crossectional TEM samples.
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EXPERIMENTAL SECTION GaN nanowire growth was carried out in a Thomas Swan showerhead 3 × 2 MOCVD cold-wall reactor. The growth substrates were prepared by thermal evaporation of gold and nickel on r-plane and c-plane α-Al2O3 wafers purchased from Monocrystal and (100) γ-LiAlO2 substrates from MTI Corporation. Hydrogen at 30 sccm was bubbled through TMG and diluted with 20 sccm hydrogen. The TMG bubbler was kept at 5 °C, and the reactor pressure was set to 140 mbar. Ammonia at 500 sccm was used, with an additional 450 sccm of hydrogen or nitrogen as the carrier gas. Growth temperature was 780° or 820 °C. A range of catalyst compositions and thicknesses were used. In general, m-axis GaN nanowires were obtained for a fairly wide range of compositions: from 14 to 86 at% nickel and a total thickness of 0.4−1.3 nm using higher nitrogen partial pressures. a-axis wires were obtained for a narrow range of compositions: from 92 to 96 at% nickel and a total thickness of 0.4−0.8 nm at 820 °C using higher hydrogen partial pressures. The TEM characterization has been carried out on a JEOL 2100F electron microscope operated at 200 kV equipped with Oxford INCA energy dispersive electron X-ray spectrometer and Tridiem Gatan imaging filter and spectrometer. Crossectional polishing and ion mill at facilities at NCEM were used for cross-sectional sample preparation. CL measurements were performed using a Zeiss Supra 55 VP-FESEM integrated with home-built, fiber-based light collection system. The spectra were acquired using a SP2300i Acton spectrometer (Princeton Instruments) equipped with
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