PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis, characterization and photoconductivity of highly crystalline InP nanowires prepared from solid hydrogen phosphide† Teck H. Lim, Shrividya Ravi, Christopher W. Bumby, Pablo G. Etchegoin and Richard D. Tilley* Received 5th February 2009, Accepted 23rd April 2009 First published as an Advance Article on the web 28th May 2009 DOI: 10.1039/b902474c Highly crystalline InP nanowires (InP NWs) were synthesized in solution via solution–liquid–solid (SLS) growth from In and hydrogen phosphide (PH)x generated in situ from controlled hydride reduction of InCl3 and PBr5 and the photoluminescence and photoconductivity properties of the nanowires deposited as solid films were investigated.
Introduction InP is a direct gap III–V semiconductor that has widespread applications in today’s optoelectronic and telecommunications industries. With a bandgap of 1.35 eV at room temperature that is well matched to the solar spectrum, epitaxial InP is integral to several record efficiency solar cell architectures.1 InP nanowires (InP NWs) exhibit unique physical properties not found in their bulk counterpart. A combination of new physical properties and large surface-to-volume ratios renders InP NWs an interesting class of semiconductor nanomaterials that are useful in the field of optoelectronic devices. Previously reported applications of InP NWs have included single photon detectors and high speed electronic devices.2,3 InP NWs have also served as a model for the study and understanding of the fundamental physics in two-dimensionally confined nanostructures.4–6 InP NWs may be fabricated from both vapour and solution synthesis routes. InP NWs prepared via vapour deposition routes such as laser-assisted catalytic growth (LCG),2,3 metal–organic chemical vapour deposition (MOCVD)6–14 and thermal evaporation deposition15,16 commonly grow via vapour–liquid–solid (VLS) growth.17–19 In solution, an analogous mechanism, first coined solution–liquid–solid (SLS) growth by Trentler and coworkers in 1995, allows InP NWs to be prepared.20 When compared to the vapour phase preparation techniques aforementioned, solution-based approaches can more readily produce the quantities of nanowires needed for large scale applications. The high solubility of surface ligand stabilizing semiconductor nanoparticles in common volatile organic solvents also offers a promising low-cost route to solution-processed crystalline semiconductor materials. Most solution syntheses of InP nanoparticles and SLS-grown InP NWs previously reported in the literature have primarily focused on the use of phosphines or organoindium complexes
School of Chemical and Physical Sciences, MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington, New Zealand. E-mail:
[email protected]. nz; Fax: +64 4463 5237; Tel: +64 4463 5016 † Electronic supplementary information (ESI) available: TEM image of Bi nanoparticles, SEM image of InP NWs as a mat and electron diffraction pattern for growth direction analysis. See DOI: 10.1039/b902474c
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containing one or more ligating phosphines as precursors.20–23 Phosphines including the commonly used tris-trimethylsilylphosphine (P(TMS)3) are intrinsically highly toxic and pyrolytic. Such high levels of hazard is detrimental to the process of scalingup preparation and subsequent mass production of InP NWs essential for economic device fabrication. Non-phosphine based phosphorus precursors are therefore attractive and there has been considerable effort to explore new phosphorus sources which are less hazardous to handle. For example, white phosphorus has been used with success in a surfactant-templated system to produce InP NWs via PH3 as an intermediate in an alkaline and pressurized environment.24 However, a non-pyrolytic non-phosphine based phosphorus source with lower toxicity has yet to be reported, thus limiting the viability of the larger scale production of phosphides via solution synthesis. PBr5 is a crystalline solid which dissolves in hexadecane to form a stable solution at room temperature. When treated with LiBH4, a yellow solid precipitates out from the solution. The yellow solid, which could be handled in air without risk of pyrolysis occurring, is similar to a solid hydrogen phosphide (PH)x which Wilberg and Muller-Schiedmayer isolated in 1959 from the reaction of LiH and PBr3 in ether.25 Solid hydrogen phosphide has been known for several decades although more commonly known as the lower hydrides of phosphorus. The solid hydrogen phosphide has been mainly prepared via the condensation or thermal decomposition of phosphines precursors.26 Hydrogen phosphide (PH)x prepared from PBr5 and LiBH4 presented in this report has the potential to serve as a less toxic and relatively non-flammable phosphorus source more suitable for scaled-up synthesis. In addition to the significantly reduced level of hazard, PBr5 costs 80% less than P(TMS)3 per gram and this makes PBr5 even more attractive as a phosphorus precursor.27 In this report, we demonstrate that highly crystalline InP NWs can be readily prepared by treating InCl3 and PBr5 with LiBH4 in the presence of pre-formed In or Bi metal nanoparticles as seeds to facilitate SLS growth (Fig. 1 and Fig. S1†). The key reactions towards InP formation include: (i) the formation of In and (PH)x from the controlled hydride reduction of InCl3 and PBr5; (ii) the formation of InP from In and (PH)x and (iii) the growth of InP NWs via the SLS mechanism from the metal seeds (Fig. 1). The best nanowires were produced when the reaction sequences and conditions were optimized, which were when hydrogen This journal is ª The Royal Society of Chemistry 2009
InP NWs film preparation Films were prepared by slowly drying a solution of InP NWs on clean sapphire substrates to form optically homogenous films. The films were later annealed under an Ar–H2 mixed atmosphere (5% hydrogen) at 600 C. Gold contacts were then deposited onto the annealed films and the resulting films were further annealed under N2 at 400 C for 5 min to ensure good electrical contact.
Characterizations Fig. 1 Schematic presentation of the formation and growth of InP NWs prepared from the hydride reduction of InCl3 and PBr5 in solution.
phosphide (PH)x and elemental In were produced simultaneously and in situ in solution at 250 C in trioctylphosphine (TOP). The In and Bi metal nanoparticle seeds were pre-synthesized by the hydride reduction of InCl3 and BiCl3 in N,N-diethylaniline or isobutylamine with trioctylphosphine oxide (TOPO) as a surfactant. The majority of the pre-synthesized In and Bi nanoparticles have diameters ranging from 50–90 nm and 30–40 nm (Fig. S2†) respectively.
Experimental All reagents were purchased from Sigma-Aldrich unless otherwise mentioned. InCl3 and BiCl3 (anhydrous, 99.9%), TOP (90%) and TOPO (90%) were used as received without further purification. Hexadecane (99%) was heated under vacuum at 160 C for at least 1 h to de-gas. Synthesis of In metal seeds InCl3 (0.1 g) was dissolved in dried N,N-diethylaniline (10 mL, BDH) under N2 at room temperature. To the resulting orange solution, LiBH4 in THF (2 M, 1 mL) was added dropwise. A black precipitate formed instantly which could be redispersed to form a homogeneous black solution.
A Philips X-ray diffractometer with a Cu Ka source was used for phase identification. The nanostructures of InP NWs were characterized using a JEOL 6700F FESEM (operating at 10 kV) and a JEOL 2011 HRTEM (operating at 200 kV). Photoluminescence measurements were conducted at 300 K using an Ar+ ion laser at 514 nm. For the photoconductivity study, a 50 W quartz halogen lamp with a dichroic filter was used as an illumination source and measurements were conducted with a Keithley 236 Source-Measure-Unit.
Results and discussion The as-synthesized InP NWs were isolated as a dark brown to black precipitate after purification and could be easily redispersed in chloroform or toluene. The powder X-ray diffraction (PXRD) pattern of the black powder is shown in Fig. 2 and indicates that cubic (zinc blende) InP was the only crystalline species present. InP NWs prepared from Bi seeds are long and straight and have an average diameter of 60 10 nm and an aspect ratio of 30 (Fig. 3). InP NWs grown from In seeds are shorter and typically have diameters of 50–100 nm although some as small as 15 nm were also observed. Typical aspect ratios of 14 were found (Fig S3†). The 15 nm wide nanowires were likely to have grown from a small amount of In nanoparticles with sizes around 15 nm formed during the reaction (in contrast to the larger pre-synthesized In seed nanoparticles). This suggests that if In or Bi nanoparticles of less than 20 nm in diameter are used as
Synthesis of InP NWs Freshly prepared In or Bi seed solution (0.25 mL) was added to a solution of InCl3 in TOP (0.045 M, 6 mL) at 250 C under N2. The resulting product was a light brown dispersion. To this, a red solution of PBr5 in hexadecane (0.281 M, 1 mL) and, from a separate syringe, a suspension of LiBH4 in TOP (2 M, 1 mL) were added dropwise in an alternating fashion. As the reaction proceeded, the colour of the solution mixture changed from light brown to deep brown and finally black after 1 h. The mixture was allowed to react further for another 2 h before rapid cooling to room temperature and was then subsequently purified in air. To purify, the precipitate was dispersed in toluene (25 mL) and the solution was centrifuged at 4000 rpm for 5 min. The washing process was performed four times to remove excess TOP and hexadecane. The precipitate isolated after the toluene washes was further washed with methanol (100 mL) to remove LiCl. The final precipitate was dispersed in toluene or chloroform (10 mL). This journal is ª The Royal Society of Chemistry 2009
Fig. 2 PXRD pattern of InP NWs indexed as cubic InP.
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Fig. 3 InP NWs synthesized from In and (PH)x seeded by Bi nanoparticles. Nanowires are straight and long and have an average aspect ratio of 30 and diameters ranged between 40–70 nm.
pre-synthesized seeds, InP NWs with a diameter of less than 20 nm (Bohr radius of InP) may be prepared using this method. Bi-seeded InP NWs were found to have an average diameter which is appreciably larger than the diameter of the seeds. This indicates that radial growth is significant in the Bi-seeded system. In contrast, radial growth appears to be less prominent in the Inseeded system because the InP NWs produced have diameters that are close to the diameters of In seeds. Bi seeds have been previously reported to be better than In seeds for SLS growth of nanowires.21 In our system, Bi seeds also appear to be superior to In seeds for producing InP NWs with larger aspect ratios and a tighter diameter size distribution. From HRTEM studies, both single crystalline nanowires and nanowires with twin boundaries normal to the axial growth direction (Fig. 4(a) and (b)) were observed. From the analysis of HRTEM images and electron diffraction patterns (Fig. S4†) the nanowires were found to grow along the [111] direction as has been commonly found in earlier reports.2–6,21 To facilitate further characterization, a given volume of InP NWs solution (In-seeded) was dried on a sapphire substrate in air in a confined space slowly overnight to obtain an optically homogeneous film of InP NWs. An absorption band-edge transition was observed at 1.38 eV (900 nm) for the film of InP NWs. Fig. 5(a) shows a photoluminescence spectrum measured for the same film at 300 K. The emission (photoluminescence) peak is broad, with an emission maximum at 1.66 eV (747 nm) and a FWHM of 0.38 eV. The luminescence signal is blue shifted relative to the absorption spectrum. This implies that the dominant electronic transition responsible for the absorption spectrum is different to the transition that leads to the luminescence signal. The luminescence signal is also blue shifted from that of bulk InP (direct gap of 1.35 eV). The blue shift relative to the bulk is unlikely to be the result of quantum confinement of the electrons, since the majority of the InP NWs have diameters significantly larger than the excitonic Bohr radius of InP (20 nm) and may originate from surface state emission.5,14 The photoluminescence properties of InP NWs with diameters larger than the 20 nm Bohr radius of InP have been previously reported by several groups. A common observation amongst 4854 | J. Mater. Chem., 2009, 19, 4852–4856
Fig. 4 HRTEM images. (a) A single nanowire with twin boundaries normal to the growth axis of [111] and (b) a close up view at the section in between two twin boundaries with atomic details.
these studies is a significant blue shift up to around 1.5 eV in photoemission relative to bulk InP that is not due to quantum confinement. Various explanations have been suggested to account for the commonly observed blue shift, including the presence of twin boundaries14,28,29 and the Coulomb interaction between exciton electrons and holes with charges and dipoles distributed on the surface of the nanowire.30 A small but significant blue shift has also been observed for a single crystalline zinc blende InP NW of 50 nm in diameter grown in the (111) direction.5 However, no explanation was offered for the origin of the shift. To illustrate the potential of the nanowires for photovoltaic devices, photoconductivity measurements were conducted on optically-homogeneous films made of InP NWs. Prior to photoconductivity measurements, the film was thermally treated at 600 C in a 5% H2–Ar mixture to remove organic surfactants in order to reduce the resistivity of the film. Gold contacts were then evaporated onto the film with the aid of a mask. SEM studies of the annealed films (after removal of surfactant) showed that the films retained their entangled morphology. An annealing temperature of 600 C was found to be the highest annealing temperature applicable before the entangled morphology was destroyed. This journal is ª The Royal Society of Chemistry 2009
employing pre-synthesized In and Bi nanoparticles as seeds to promote SLS growth. Bi-seeded InP NWs are straight, long and have a tight diameter distribution with an average diameter of 60 nm and an aspect ratio of 30. In-seeded InP NWs have a broader diameter distribution with diameters ranging from between 50 to 100 nm and an average aspect ratio of 14. This synthetic approach is considerably less hazardous and more economic than common phosphine-based approaches. Optical studies on the pre-annealed films of In-seeded InP NWs showed that the sample exhibits photoluminescence peaking at 1.66 eV at 300 K, blue shifted with regard to bulk InP. Annealed films were found to exhibit a 20 fold increase in photoconductive current under illumination of a 50 W white light source (compared to that in darkness). Further studies on fine tuning the method to produce InP NWs with diameters smaller than the Bohr radius of InP (20 nm), and extending the methodology to produce other phosphide nanostructures are currently in progress. Hydrogen phosphide (PH)x prepared from PBr5 represents a new non-pyrolytic phosphorus source with lower toxicity suitable for the larger scale synthesis of a whole range of significant semiconductor inorganic phosphides.
Acknowledgements Fig. 5 Optical and electrical characterizations. (a) Photoluminescence spectrum of InP NWs in the form of a thin film. (b) On–off characteristics of the InP NWs film.
Fig. 5(b) shows the results of the conductivity measurements made at 293 C on a typical annealed film. The films were observed to exhibit ohmic conduction at applied fields below 8 104 V cm 1. Typical sheet resistances of 2 GU , 1 were observed under dark conditions. Photocurrent generation was observed under focused illumination from a filtered 50 W quartz halogen lamp emitting between 400 and 750 nm. The photocurrent led to an increase in sheet conductance by a factor of 20. Repeated light–dark cycles on the sample were reproducible and showed a delayed photo-response for both the rising and falling photocurrent (Fig. 5(b)). This occurred over a time-scale of several minutes. The transient photocurrent rises exponentially with time following sudden illumination under a constant bias voltage and the time constant is significantly longer than expected for transient RC damping due to stray capacitance in the measurement circuit. These findings are consistent with the filling of electronic trap states by photogenerated charge carriers generated at a constant rate.31 Such behavior can be expected in the ‘‘strong electron-trapping’’ regime whereby charge carriers are initially trapped at a discrete state below the Fermi level.29 The trap states empty through thermal excitation processes and carriers are re-trapped on multiple occasions before being swept out of the region of applied field.
Conclusions In summary, highly crystalline InP NWs were prepared via the reaction of elemental In and hydrogen phosphide (PH)x This journal is ª The Royal Society of Chemistry 2009
R. D. T., C. W. B. and P. G. E. thank the MacDiarmid Institute for Advanced Materials and Nanotechnology for funding. T. H. L. thanks the New Zealand International Doctoral Research Scholarship for funding. R. D. T. and T. H. L. thank Prof. Richard J. D. Tilley for useful discussions.
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