7212
J. Phys. Chem. 1996, 100, 7212-7219
Synthesis of Size-Selected, Surface-Passivated InP Nanocrystals A. A. Guzelian, J. E. B. Katari, A. V. Kadavanich, U. Banin, K. Hamad, E. Juban, and A. P. Alivisatos* Department of Chemistry, UniVersity of California, Berkeley, California 94720, and Materials Science DiVision, Lawrence Berkeley Laboratory, Berkeley, California 94720
R. H. Wolters, C. C. Arnold, and J. R. Heath* Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095-1569 ReceiVed: December 13, 1995; In Final Form: February 5, 1996X
Quantum-confined InP nanocrystals from 20 to 50 Å in diameter have been synthesized via the reaction of InCl3 and P(Si(CH3)3)3 in trioctylphosphine oxide (TOPO) at elevated temperatures. The nanocrystals are highly crystalline, monodisperse, and soluble in various organic solvents. Improved size distributions have been obtained by size-selectively reprecipitating the nanocrystals. The UV/vis absorption spectra of the particles show the characteristic blue shift of the band gap of up to 1 eV due to quantum confinement, a moderately well-resolved first excitonic excited state, and, in some cases, the resolution of a higher excited state. Structurally, the nanocrystals are characterized with powder X-ray diffraction and transmission electron microscopy. Raman spectroscopy reveals TO and LO modes near the characteristic bulk InP positions as well a surface mode resulting from finite size. The Raman line widths, line positions, and relative intensities are all size-dependent . X-ray photoelectron spectroscopy (XPS) shows the nanocrystals have a nearly stoichiometric ratio of indium to phosphorus with TOPO surface coverages ranging from 30% to 100%. We have also used XPS to correlate the oxidation of the nanocrystal surface with photoluminescence intensity. Photoluminescence is observed as both band edge and deep trap emission with both features shifting with nanocrystal size. The luminescence is highly dependent on the surface of the nanocrystal with oxidation being a necessary condition for emission.
I. Introduction The production of nanocrystals of inorganic semiconductors remains a major stumbling block for the investigation of quantum size effects. While the II-VI materials CdSe and CdS have been studied extensively,1 synthetic limitations have precluded similar treatment of other materials. Recently, however, papers discussing the synthesis and characterization of GaAs2 and InP3 nanocrystals have begun to appear in the literature. The chemical nature of III-V nanocrystals is expected to be quite different from the more ionic II-VI prototypical systems. A method for producing narrow size distributions of a III-V semiconductor nanocrystal system over a broad size range and as a colloidal material would facilitate comparative studies of nanocrystals, thus providing an enhanced understanding of quantum confinement effects. The III-V nanocrystals are expected to exhibit quantum size effects (e.g., blue shifts in the absorption spectrum and the development of discrete electronic structure) in full analogy with the prototypical II-VI materials. They may even exhibit more pronounced effects due to differences in their respective bulk properties.4 Maximum quantum size effects are expected from materials that allow for larger delocalization of the electron and hole.5 Delocalization is enhanced by higher dielectric constants, which allows more complete screening of the Coulomb attraction between the electron and hole, and smaller effective masses and weaker phonon coupling, which allow for easier movement of the carriers through the lattice. Group III-V materials, with their relatively covalent bonding and direct gap band structure (except GaP), best combine these characteristics and would seem to be the most promising candidates for large quantum confineX
Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-7212$12.00/0
ment effects. The bulk exciton diameter gives some sense of this; for InP it is 150 Å while for CdSe it is only 70 Å. In a progressively more covalent series of semiconductors, the synthesis of nanocrystals by colloidal techniques becomes increasingly difficult. In the case of I-VII or II-VI nanocrystals, bare atoms or ions can be used as precursors, allowing for the direct reaction of the relevant elements. This is key to controlling the nucleation and growth steps,1a as well as ensuring that many bonding geometries can be sampled during the growth stage. In contrast, the corresponding III-V atoms or ions, if used as precursors, may directly react with the solvent. Organometallic precursors of III-V crystals offer lower reactivity than atoms or ions, but the chemical pathways to crystal formation are typically characterized by comparatively large reaction barriers, thereby complicating the sequence of nucleation and growth. Without a temporally discrete nucleation step, the production of monodisperse nanocrystals with a narrow size distribution is difficult.6 Several groups have demonstrated organometallic routes for the fabrication of III-V materials.2d,7 Such schemes usually lead to nanocrystalline powders rather than discrete nanocrystalssa direct result of the difficulty involved in separating and controlling the nucleation and growth steps. Very recently, Nozik et al.3a made a large step forward with their adaptation of Wells’ dehalosilylation reaction8 to produce InP nanocrystals with the necessary characteristics of high crystallinity, monodispersity, and solubility. In this paper, we have modified their synthesis and employed the tools of surface modification and size-selective precipitation1a with the specific goals of controlling nanocrystal size distributions and nanocrystal surface passivation. We have thoroughly characterized these particles. Particle size, crystallinity, lattice structure, © 1996 American Chemical Society
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morphology, and size distributions were measured using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) has been used to evaluate the composition of the samples and to examine the surface of the particles. UV/ vis absorption spectroscopy and photoluminescence (PL) spectroscopy have provided information on the shift in the particles’ band gap due to quantum confinement effects and how this shift varies with particle size. Correlation of XPS data with PL efficiencies indicates that surface oxidation is critical for efficient PL. II. Experimental Section A. Synthesis of InP Nanocrystals. If these InP nanocrystals are to be compared with the best II-VI nanocrystals, certain chemical characteristics are critical, including high crystallinity, monodispersity, narrow size distributions over a broad range of sizes, and high solubility in organic solvents. We have used the following reaction scheme and size selective precipitation to meet these criteria: 100 °C, 12 h
InCl3+ TOPO 98 +P(Si(CH3)3)3, 265 °C, days
In-TOPO complex 98 + surfactant (S), 100 °C, days (S ) RNH2, RSH, RPH2)
InP (TOPO capped) ssssssssssssssssssf InP (capped by S and TOPO) (1) A typical synthesis is as follows: 0.25-0.35 g of InCl3 is added to 25 mL of TOPO in the drybox, giving a clear, colorless solution. This solution is stirred under argon at 100 °C for 12 h. An equimolar amount of neat P(Si(CH3)3)3 is then added at 100 °C via syringe. Upon injection the solution turns bright yellow. This mixture is maintained at 100 °C for 3 h during which time the color evolves to a clear orange/red. The temperature is then raised to 265 °C over 2 h. As the temperature increases, the color changes through increasingly dark red to a dark brown. Throughout the reaction all reactants and products stay in solution, and any sign of a precipitate indicates that the reaction has failed. The reaction is maintained at 265 °C for 6 days. To add the surface cap, the solution is cooled to 100 °C, and a quantity of the desired capping molecule (0.5 mL of dodecylamine, for example) is injected. The reaction is maintained at 100 °C for 3 days and then at 60 °C for two additional days. It should be noted that crystalline particles may be obtained after 1-2 days. However, we have found that the full annealing sequence yields nanocrystals with the best range of sizes, optical properties, and solubility characteristics and which selectively precipitate to give the narrowest size distributions. All reactions were carried out using standard airless techniques. Anhydrous reagents and unoxidized nanocrystals were stored in a nitrogen-filled drybox. InCl3 (99.999%) was purchased from Strem Chemical Co. and used as received. P(Si(CH3)3)3 was synthesized according to literature methods,9 purified by vacuum distillation, and stored at -25 °C in the drybox [1H NMR (400 MHz, CDCl3) δ 0.3 (s, 9 H)]. Trioctylphosphine oxide (TOPO) (90%, Aldrich) was purified via vacuum distillation, retaining the fraction transferred between 190 and 200 °C at 60 mTorr. Anhydrous toluene was distilled over sodium. Anhydrous methanol was purchased from Aldrich. Size-SelectiVe Separation and UV/Vis of the Nanocrystals. For oxidized nanocrystals, the following procedure can be carried out in air. To maintain an unoxidized nanocrystal
surface, the size separation is carried out in the drybox using anhydrous solvents. However, excellent separations (of partially oxidized particles) may be carried out in air. Toluene is added to the final reaction mix, roughly doubling the volume and giving a dark brown solution. Methanol is added until the solution becomes cloudy. This first precipitate is generally a TOPO byproduct and is discarded. Methanol is then added incrementally, and the solution is filtered after each addition, isolating a narrowed size distribution of nanocrystals, which become successively smaller throughout the precipitation series. This can be repeated up to 40 times on a single reaction mixture if small enough volumes of methanol are used, until no color remains in the solution. Structural analysis indicates that a sufficiently careful precipitation series can resolve size distributions separated by as little as 1.5 Å. The precipitated fractions are immediately redissolved in toluene, although the nanocrystals are also soluble in a variety of other solvents including hexane, carbon tetrachloride, benzene, diethyl ether, glyme, chloroform, and pyridine. Room temperature UV/vis absorption spectra of InP particles dissolved in toluene were obtained immediately upon separation using a Perkin-Elmer Lambda 3B UV/vis spectrophotometer or a Hewlett-Packard 8452A diode array spectrophotometer with the samples in 1 cm quartz cuvettes. Absorption spectra were also obtained at 78 K for several of the dodecylamine-capped samples. For this, the particles were deposited in an optically clear poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (pvba) matrix on a sapphire wafer. The wafer was mounted on the copper base of an liquid nitrogen cooled optical Dewar, and absorption of light from a quartz tungsten halogen lamp was monitored using an Oriel 257 monochromator and normalized to a pvba blank. B. Structural Characterization. Transmission Electron Microscopy. Average nanocrystal sizes and distributions were measured by TEM on a TopCon EM002B electron microscope with the ultrahigh-resolution (UHR) objective lens pole piece installed. To minimize beam damage, the microscope was operated at an accelerating voltage of 120 kV, though some sample degradation still resulted in In2O3 formation. Samples were deposited from solution onto thin amorphous carbon films supported by either standard high-mesh (500 or 600) 3.0 mm diameter copper transmission electron microscope grids or copper grids coated with holey carbon films. A drop of toluene solution was placed onto the grid, excess material was wicked away, and the grid was air-dried. For larger nanocrystal sizes, centered dark-field (CDF) images were recorded between 400K× and 550K× magnification (K× ) 1000 times), using exposure times of 25 or 40 s. Magnifications were calibrated using high resolution TEM (HRTEM) images of Si nanocrystals taken under the same conditions. The internal precision of these calibrations was about 1%. A total of 100-200 particles were sized for each sample. For the smaller crystallite sizes HRTEM images were collected. The magnifications were calibrated as above using Si nanocrystals. Typically, negatives were exposed for 1-2.5 s. Particle sizes were measured directly off the negatives by counting fringes in one direction and by measuring the maximum length of fringes in the perpendicular direction. The calibration was sufficiently precise to distinguish InP nanocrystal fringes from In2O3 fringes. Up to 100 measurements were obtained for each sample. Powder XRD. Spectra of the dodecylamne-capped particles were taken on a Siemens D5000 spectrometer using Cu KR radiation. An exit slit of 1 mm was used to maximize the signal. Samples were prepared by removing the toluene and dispersing
7214 J. Phys. Chem., Vol. 100, No. 17, 1996 the sample in methanol. The dispersion was centrifuged, the methanol removed, and the resulting powder dried. The powder was then spread on a Si(100) wafer or glass plate for analysis. Domain sizes of nanocrystals were obtained by fitting the width of the peaks and applying standard formula for spherical particles.10 Raman Spectroscopy. A toluene solution of dodecylaminecapped InP particles was evaporated on the face of a sapphire window, which was then mounted on the Cu cold finger of a liquid nitrogen cooled optical cell. Resonance Raman spectra were recorded using the 647.1, 568.2, and 514.5 nm lines (2050 mW laser power) of a Coherent multigas ion laser. Raman spectra were not observed if the excitation laser frequency was below the liquid nitrogen band gap of the particles being probed. Plasma lines were removed via a Pellen-Broca prism with a spatial filter of 3 m and using a double-band-pass interference filter with a 3 nm bandwidth. A holographic notch filter with a spectral bandwidth of 345 cm-1 centered at the laser line was placed at the entrance slit of the monochromator to remove signal from Raleigh scattering. Raman spectra were dispersed through an ISA 1 m focal length, f/6.8 single Czerny-Turner type monochromator with 2400 grooves/mm grating and an entrance slit width of 50 µm. The light was dispersed on an EG&G liquid nitrogen cooled 1024 × 256 pixel CCD camera. The resolution of the instrument is better than 0.5 cm-1 at 600 nm. Spectra were collected for up to 400 30 s integrations (∼4 h) and calibrated against rare gas emission lamps. For several samples, spectra were collected at various laser powers to ensure that thermal broadening from laser light absorption by the particles was not contributing to the line widths or shifts. The Raman spectra were superimposed on a strong fluorescence background. A linear curve was therefore subtracted from the collected spectra prior to statistical analysis of the phonon features. C. Stoichiometry and Surface Composition of InP Nanoparticles. XPS. To obtain the XPS of the samples, the particles were bound to gold surfaces via hexanedithiol linkages using techniques similar to those described elsewhere.11 Briefly, hexane dithiol is first allowed to stick on ion-etched gold-coated aluminum plates. The gold substrates, which prevent charge buildup in the particles, are then transferred to an oxygen-free environment and placed in a solution of nanocrystals. On the basis of previous experience with CdS and CdSe nanocrystals, we believe that this method results in the formation of less than a single monolayer of InP nanocrystals covalently bound to the gold film. Unoxidized samples tend to bind more readily than the oxidized samples. XPS was performed using a Perkin-Elmer PHI 5300 ESCA system. Mg and Al anodes driven at 400 W were used for initial identification of the elements. Quantitative scans were obtained from the Mg anode exclusively. All scans were obtained using a 1 cm2 aperture. Survey scans were collected over the range from 1100 to 0 eV with 179 eV pass energy detection (4.5 eV resolution) typically for 3 min. Higher resolution scans were collected over a range of 20 eV around the peak of interest with either 8.9 or 35.7 eV pass energy detection (0.69 and 1.1 eV resolution, respectively), typically for 10-20 min per element. Measurements were performed at pressures ranging from 3 × 10-9 to 1 × 10-8 Torr. Spectra were calibrated using the Au 4f peak position present from the Au substrate. In addition, spectra of bulk InP were collected once for each set of measurements in order to maintain consistency between different data sets. Photoluminescence. Room temperature photoluminescence (PL) measurements were performed in a 1 cm cuvette. Con-
Guzelian et al. tinuous wave (CW) PL spectra were recorded using the 514.5 nm line of a Spectra Physics Ar+ ion laser for excitation. The collected emission was dispersed in a 0.25 m monochromator (Spex 1681B) and detected using a lock-in amplifier (EG&G 5209) with a photomultiplier tube (Hamamtsu R1477) for the visible region and a liquid nitrogen cooled Ge detector (North Coast) for the near-IR region. All spectra were corrected using a calibrated source. Quantum yields were determined by comparison with a Rhodamine 590 or Rhodamine 640 dye standards. III. Results and Discussion Synthesis and Passivation of InP Nanocrystals. The synthesis described here is a slow process in which nucleation and growth occur simultaneously over long time scales, resulting in broad particle size distributions. The particles are, however, highly crystalline and exhibit excellent solubility in organic solvents. The solubility facilitates subsequent size separation via the reprecipitations. The passivation of the particle surfaces is accomplished initially by the coordinating solvent medium of the hightemperature reaction. We have found that the solvents for (1), TOPO (and TOP), are effective in passivating the particle surface. Similar results have been observed for the case of CdSe nanocrystals.1a Based on analogy to CdSe and standard donoracceptor analysis, the TOPO would coordinate to acceptor surface indium sites, providing a passivating shell to terminate growth. In addition, TOPO would prevent agglomeration among particles and, because of its alkyl groups, provide for excellent solubility in organic solvents such as toluene and hexanes. Once the nanocrystals are stabilized in the initial reaction solvent, their surfaces are modified with a variety of passivating ligands including amines, thiols, and phosphines. Dodecylamine was found to be effective in stabilizing the nanocrystals as measured by their degree of solubility and by our ability to obtain narrow size distributions from various samples. 1H NMR on nanocrystals with surfaces modified by dodecylamine indicate the presence of both TOPO and dodecylamine. The resonances are substantially broadened compared with the free molecules, indicating a reduction in rotational freedom associated with being bound to the nanocrystal surface. Other indications that the additional passivating ligands are modifying the surface are definite changes in the solubility characteristics of the particle and by the observation of a white precipitate following the addition of the passivating ligand, indicating the replacement of some of the TOPO material initially on the nanocrystal surface. In particles with further surface derivitization, washing in pure methanol results in a permanent loss of solubility, while those particles capped only with TOPO from the initial reaction maintain their solubility in nonpolar solvents after identical treatment. Size Separation and UV/Vis Spectroscopy. If the particles are consistently capped with, for instance, dodecylamine, then the solubility of the particles should directly reflect the particle surface area and, hence, size.1a,12 The nanocrystals described here are soluble in toluene and insoluble in methanol. The stepwise addition of methanol to the reaction solution results in the incremental size-selective precipitation of the nanocrystals, with the largest particles precipitating first. In this way we have isolated 50-20 Å diameter nanocrystals, all from the same reaction mixture. The first fractions removed from the reaction mixture give dark brown solutions while subsequent fractions slowly evolve through brown/green and finally pink for the smallest sizes. This is an indication of the shifting band gap as the degree of
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Figure 1. UV/vis absorption spectra for a series of nanocrystal sizes. The bulk InP band gap is at 1.35 eV.
quantum confinement is increased with decreasing size. The UV/vis spectra shown in Figure 1, which correlate to particle sizes ranging from near 50 Å (bottom) to just less than 20 Å , give immediate, quantitative feedback regarding the effectiveness of the size separations. These spectra were taken from every second to fourth fraction removed from a dodecylaminepassivated reaction mixture. By correlating XRD measured sizes (see below) with the selective precipitation procedure, it was found that the resolving power of the precipitation technique is about 1-1.5 Å. The particles exhibit band gaps ranging from 1.7 eV for the largest size to 2.4 eV for the smallest. Compared with the bulk InP band gap of 1.35 eV, even the largest nanocrystals exhibit marked quantum confinement. The extent of the size separations is apparent from the width of the feature at the absorption onset, corresponding to the first electronic excited state. In several samples, a higher excited state is also resolved, which may be from light hole formation. In the smallest particles, the first excited state appears broadened. Whether this phenomenon results from relatively broader distributions in the final precipation steps or not is unclear. It is unlikely that the first excitonic state actually broadens as particle size decreases. The absorption thresholds in spectra obtained at 78 K for sizes ranging from 27-23 Å exhibit a blue shift from the room temperature absorption edge by 0.08-0.1 eV. This is close to the 0.07 eV band edge shift observed for bulk InP over the temperature range 0-300 K. B. Structural Characterization. Transmission Electron Microscopy. TEM images show that the nanocrystals are highly crystalline and monodisperse. A typical image is shown in Figure 2. Lattice fringes are clearly observed which index to the InP (111) lattice spacing. While the size distributions are the narrowest yet obtained for III-V nanocrystals, they are still relatively broad ((20% standard deviation) when compared with the best CdSe nanocrystals ((5% standard deviation). These nanocrystals were found to be slightly ellipsoidal with an aspect ratio varying between 1 and 1.15. Most particles were free of defects. Nanocrystal size determinations were made using two methods. In one method calibrated high-resolution
Figure 2. Transmission electron micrograph of InP nanocrystals.
Figure 3. Excitonic peak position from UV/vis vs nanocrystal diameter as determined by powder TEM (circles) or XRD (squares).
TEM images were used to obtain sizes by counting the lattice fringes of the nanocrystals in one direction and measuring the length of the fringes in the perpendicular direction. The second method used calibrated centered dark field (CDF) images from which particle sizes were measured directly. These size determinations were correlated with the shift in the band gap and are plotted as the open circles in Figure 3. Powder XRD. The XRD patterns of a series of nanocrystal sizes separated out from a dodecylamine reaction mix are shown in Figure 4. Each of the spectra, to within experimental error, fit to the bulk InP zinc-blende lattice constant of 5.87 Å. Domain sizes were extracted from these spectra by fitting the observed (111), (220), and (311) diffraction peaks to Lorentzian line profiles and averaging the domain size calculated for each
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Figure 6. XPS survey spectra for 44 Å diameter InP nanocrystals bound to a gold surface.
Figure 4. Powder X-ray diffraction of a series of nanocrystal sizes. The stick spectrum gives the bulk reflections with relative intensities.
Figure 5. Resonance Raman spectra for several nanocrystal sizes. The TO and LO modes are found at approximately 311 and 347 cm-1, respectively. The surface mode is near 330-335 cm-1.
peak width using the well-known Debye formula.10 The relatively weak (200) and (222) diffraction peaks were not used for size determination. Domain sizes compare well with TEM size distributions, consistent with the lack of defects observed in TEM, and sizes determined in this manner are included as solid squares in Figure 3. While there is a large amount of scatter in the data, the points appear to follow the inverse square curve. This may be fortuitous; generally, the effective mass model fails at even moderately small nanocrystal sizes. Empirically, the fit gives a crude method for estimating nanocrystal size from optical absorption spectra. Raman Spectroscopy. Figure 5 shows resonance Raman spectra for the same particle series giving the XRD data shown in Figure 3, with particle sizes determined from the XRD
analysis. Each spectrum is characterized by three features: the transverse optical (T0) mode near 310 cm-1, the longitudinal optical (LO) mode near 347 cm-1, and a surface mode (S) near 330-335 cm-1, although for the smallest particles the LO and S features are indistinguishable. Both the LO and TO modes are observed in bulk InP, and the S mode is a result of finite size. Various discussions of the S mode as it relates to quantum well structures and discharge-produced quantum dots are available in the literature.13 It is interesting to note that the TO mode is never observed in II-VI quantum dots, although it is observed in the bulk for those systems as well. Several interesting size-dependent features in the spectra are worth noting. As particle size decreases, the line widths broaden and the LO and TO frequencies converge slightly.14 The broadening is consistent with phonon confinement models, and a lower polarizability in the smaller particles may account for the frequency convergence. In addition, the relative intensity of the T0 mode increases with decreasing particle size, changing from just 20% of the LO intensity for the 43 Å particles to near 67% for the
