Solution Phase Synthesis of Indium Gallium Phosphide Alloy Nanowires ^ Nikolay Kornienko,† Desire´ D. Whitmore,† Yi Yu,† Stephen R. Leone,†,‡, ) and Peidong Yang*,†,§, ,# †Department of Chemistry, ‡Department of Physics, and §Department of Materials Science Engineering, University of California, Berkeley, California 94720, United ^ States, Chemical) Sciences Division and Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, California 94720, United States, and #Kavli Energy Nanosciences Institute, Berkeley, California 94720, United States ABSTRACT The tunable physical and electronic structure of IIIÀV semiconductor alloys renders them uniquely useful for a variety of applications, including biological imaging, transistors, and solar energy conversion. However, their fabrication typically requires complex gas phase instrumentation or growth from high-temperature melts, which consequently limits their prospects for widespread implementation. Furthermore, the need for lattice matched growth substrates in many cases confines the composition of the materials to a narrow range that can be epitaxially grown. In this work, we present a solution phase synthesis for indium gallium phosphide (InxGa1ÀxP) alloy nanowires, whose indium/gallium ratio, and consequently, physical and electronic structure, can be tuned across the entire x =0to x = 1 composition range. We demonstrate the evolution of structural and optical properties of the nanowires, notably the direct to indirect band gap transition, as the composition is varied from InP to GaP. Our scalable, low-temperature synthesis affords compositional, structural, and electronic tunability and can provide a route for realization of broader InxGa1ÀxP applications. The IIIÀV semiconductors are well sui-ted lighting applications because this range for a number of optoelectronic encompasses the majority of the visible applications because of their typically light spectrum.12 The ability to synthe- high carrier mobilities and direct band size InxGa1ÀxP in the nanowire (NW) mor- gaps.1 Among them, pseudobinary alloys phology with tunable structural and elec- of IIIÀVs feature structural and electronic tronic properties can pave a route toward properties such as band gap, index of potential applications in miniaturized op- refraction, and lattice constant that can be toelectronic devices, tunable NW lasers, fi directly modi ed through the tuning of and high surface area photocatalysts. 2,3 their composition. Due to this unique However, there are several drawbacks control, IIIÀV alloys find special uses in bio- that impede large-scale implementation 4 5 fi medical imaging, light-emitting diodes, of this material: bulk InxGa1ÀxP, thin lms, 6 7 lasers, heterojunction bipolar transistors, or nanostructures are typically synthe- and high-performance multijunction photo- sized through liquid phase epitaxy (LPE), 8 voltaic cells. metalÀorganic chemical vapor deposi- Indium gallium phosphide (InxGa1ÀxP) is tion (MOCVD), or molecular beam epitaxy À a pseudobinary III V alloy of an indirect (MBE) techniques, which are coupled band gap semiconductor, gallium phos- with several inherent limitations, includ- phide (GaP), and a direct band gap semi- ing low material yield and expensive 13 conductor, indium phosphide (InP). This growth processes. Furthermore, a com- alloy possesses a tunable band gap that mon requirement for lattice matched sub- ranges from 1.35 to 2.26 electron volts (eV), strates such as gallium arsenide (GaAs) or which is direct for the composition range germanium (Ge) limits the composition 9À11 1 g x g 0.2À0.3. The band gap range range of the resulting materials to compo- of 1.35À2.26 eV is particularly attractive sitions that can grow epitaxially on these for use in solar energy conversion and substrates. Figure 1. Synthetic scheme (A) of the InxGa1ÀxP nanowire growth. SEM (B) of InxGa1ÀxP NWs with x = 0.50 drop cast on a silicon substrate. TEM images of NWs (CÀH) with composition x = 0, 0.21, 0.41, 0.66, 0.84, and 1.0, respectively. RESULTS AND DISCUSSION In this work, we present a scalable, low-temperature solution phase synthesis for InxGa1ÀxPNWs withfull We synthesized InxGa1ÀxP NWs with a one-pot, self- compositional, and therefore, structural and electronic seeded SLS synthesis (Figure 1A) in a nitrogen envi- tunability. In the recent literature, InP and GaP NWs ronment using standard Schlenk line techniques. In were synthesized in the solution phase through a this process, a liquid metal seed in solution serves as variety of techniques, including electrodeposition14 both the growth catalyst and group III precursor 15 and synthesis in aqueous solvents, supercritical source. Trimethylindium (TMIn) and triethylgallium fl 16 17À24 uids, and organic solvents. Moreover, (TEGa) precursors were injected into a room-tempera- 25 À AlxGa1ÀxAs alloy NWs and various II VI NW alloys 26À28 ture, noncoordinating squalane solvent, and the solu- and heterostructures have also been synthesized tion was stirred until homogeneous. Next, the reaction in the solution phase. InxGa ÀxP quantum dots have 1 ° mixture was lowered into a 280 C molten salt bath and been synthesized through the thermal decomposition 29 heated for 60 min to fully decompose the metal of precursors in an organic solvent. InxGa1ÀxP NWs, precursors and generate homogeneous indiumÀ however, have been synthesized only through vapor 30À34 gallium alloy growth seeds. The In/Ga seed solution phase CVD and MBE methods. In light of the was next removed from the molten salt and allowed previous synthesis and applications of GaP, InP, and to cool to room temperature. After cooling, tris- InxGa1ÀxP NWs, we aimed at developing a solution (trimethylsilyl)phosphine (TMSP) phosphorus precur- phase synthesis of InxGa1ÀxP NWs featuring (I) compo- sition control, (II) low-temperature growth, (III) scal- sor was then injected into the reactor, which was ° ability, and (IV) no requirement for lattice matched subsequently lowered into the same 280 C molten substrates. In the following sections, we show that a salt bath and heated for an additional 60 min. The À À preseeded solution liquid solid (SLS) synthesis can solution changed color from gray to black upon the growth of the InxGa ÀxP NWs. After thorough cleaning be used to controllably grow InxGa1ÀxP NWs across the 1 entire composition range of x =0 to x = 1. Through of the NWs through multiple centrifugation steps, systematic structural and optical measurements, we a hydrochloric acid etch removed the metal seeds. demonstrate structural and electronic tunability of Because TMIn and TEGa decompose at different rates InxGa1ÀxP NWs and provide insight into both the under our reaction conditions, forming the In/Ga disordered alloy crystal structure and the direct to growth seeds in a separate step was necessary to indirect band gap transition. obtain homogeneous InxGa1ÀxP NWs. A mixture of Figure 2. Electron microscopy analysis of InxGa1ÀxP NWs. A typical EDS spectrum (A) featuring prominent In, Ga, and P peaks and their corresponding elemental maps in the inset. EDS map of a NW and growth seed (B) reveals an In-rich seed relative to the NW composition. Z-contrast HAADF-STEM of InxGa1ÀxP NWs (C, D) with x = 0.66 and x = 0.50 features no abrupt contrast changes that would be indicative of sharp compositional fluctuations. A line scan along the length of the NW (E) shows no significant variations in the In/Ga elemental ratio though the possibility of compositional fluctuations of a few nanometers cannot be excluded. separate InP and GaP NWs instead of alloys resulted indicating the presence of both In and Ga in the NWs, when TMIn, TEGa, and TMSP precursors were mixed with EDS mapping in the inset illustrating the corre- and heated in a single step (Figure S2). sponding elements distributed throughout the NWs. Following the synthesis, we characterized the mor- EDS mapping of an individual wire and growth seed phology and composition of the NWs with electron (Figure 2B) reveals that the InxGa1ÀxP NW was more microscopy. The size and shape of the synthesized Ga-rich than the growth seed, which was observed for NWs were first studied with scanning electron micro- every batch of alloy NWs. The tendency of InxGa1ÀxPto scopy (SEM). Figure 1B shows a typical SEM image of grow as a more Ga-rich composition relative to the x InxGa1ÀxP NWs with a composition of = 0.50. Trans- starting material is commonly observed and is ex- À mission electron microscopy (TEM) in Figure 1C H plained by a lower formation enthalpy for the GaP 35À38 offers a closer look at the NWs and illustrates the relative to InP. However, the NW body was uni- morphology of several batches of NWs with different form in composition: the lack of abrupt compositional compositions. The diameters of the NWs typically variation is first evidenced in a lack of contrast in varied between 70 and 150 nm, and their lengths Z-contrast high-angle annular dark field scanning between 1000 and 2500 nm. The composition of the transmission electron microscopy (HAADF-STEM) in NWs was controlled by the TMIn:TEGa ratio of the Figure 2C and D. Furthermore, a typical elemental line precursor, and the diameter of the NWs was controlled scan (Figure 2E) indicates that the composition of the by the precursor concentration (Figure S3). NW generally remains uniform along the growth direc- To assess the chemical composition and element tion. However, with the techniques used, we cannot distribution of the resulting NWs, scanning transmis- rule out fluctuations in composition of several nano- sion electron microscopy energy dispersive X-ray spec- meters or less. troscopy (STEM-EDS) analysis was utilized. Figure 2A We proceeded to study the structural properties, shows a typical EDS spectrum taken from InxGa1ÀxP including crystal phase, growth direction, and pre- x NWs, with = 0.21, with Ga, In, and P peaks labeled, sence or lack of long-range ordering of the InxGa1ÀxP Figure 3. Powder XRD spectra (A) exhibit a shift in the (111) diffraction peak to larger 2 theta values with increasing Ga content, indicative of a lattice constant decrease. This lattice constant decrease is linear with composition, in accordance with Vegard's law (B).