Atomic Layer Deposition of Lead Sulfide Quantum Dots on Nanowire

Feb 14, 2011 - Seokhwan Bang , Seungjun Lee , Taeyoung Park , Youngbin Ko , Seokyoon Shin , Sang-Youp Yim , Hyungtak Seo , Hyeongtag Jeon...
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LETTER pubs.acs.org/NanoLett

Atomic Layer Deposition of Lead Sulfide Quantum Dots on Nanowire Surfaces Neil P. Dasgupta,*,†,|| Hee Joon Jung,‡,|| Orlando Trejo,† Matthew T. McDowell,‡ Aaron Hryciw,§ Mark Brongersma,§ Robert Sinclair,‡ and Fritz B. Prinz†,‡ †

Department of Mechanical Engineering, ‡Department of Materials Science and Engineering, §Geballe Laboratory for Advanced Materials, Stanford University, California 94305, United States

bS Supporting Information ABSTRACT: Quantum dots provide unique advantages in the design of novel optoelectronic devices owing to the ability to tune their properties as a function of size. Here we demonstrate a new technique for fabrication of quantum dots during the nucleation stage of atomic layer deposition (ALD) of PbS. Islands with sub-10 nm diameters were observed during the initial ALD cycles by transmission electron microscopy, and in situ observations of the coalescence and sublimation behavior of these islands show the possibility of further modifying the size and density of dots by annealing. The ALD process can be used to cover high-aspect-ratio nanostructures, as demonstrated by the uniform coating of a Si nanowire array with a single layer of PbS quantum dots. Photoluminescence measurements on the quantum dot/nanowire composites show a blue shift when the number of ALD cycles is decreased, suggesting a route to fabricate unique three-dimensional nanostructured devices such as solar cells. KEYWORDS: Atomic layer deposition, quantum dots, nanowires, photovoltaics, transmission electron microscopy,

There has been a recent explosion in solar cell research using nanostructured architectures to improve efficiency and minimize materials utilization. The main challenges for improving solar cell performance are maximizing absorption of photons, improving conversion efficiency of photons into excited charge carriers and extracting these carriers to the electrodes before recombination occurs. One way to improve absorption is through light-scattering and trapping, which has been observed in three-dimensional (3D) architectures such as microwire,1 nanowire,2-4 and nanocone5,6 arrays. Conversion of photon energy into excited charge carriers is typically limited by thermodynamics to the ∼31% Shockley-Queisser limit7 for single-gap cells. Lead chalcogenide quantum dots (QDs) provide several possible routes to solar cells not restricted to this limit through hot-carrier extraction,8 multiple exciton generation,9-11 and infrared absorption layers in multijunction cells.12 In practice, implementation of these devices is challenging, as charge extraction from QDs requires interdot tunneling, which needs to occur over relatively large distances in devices thick enough to absorb solar radiation. Therefore, it is desirable to coat 3D light scattering architectures conformally with layers of QDs, which would allow for a significant reduction in the required thickness of a QD absorber layer, allowing for easier extraction of excited charge carriers before recombination occurs. Here we show a new technique for deposition of QDs during the initial nucleation cycles of atomic layer deposition (ALD). This technique was used to conformally coat silicon nanowires (SiNWs) r 2011 American Chemical Society

with single layers of PbS QDs, allowing for the fabrication of device architectures that could benefit from the light-scattering ability of high-aspect ratio structures and the unique properties of QDs for next-generation solar cells. The ALD technique provides several advantages over standard QD fabrication methods. While colloidal growth allows for lowcost, high-throughput fabrication of QDs with a small size variance, the formation of dots requires the presence of surface ligands to prevent agglomeration.13 These ligands are often insulating, hindering charge extraction from the dots.14,15 They also define the quantum confinement barrier, limiting the ability to engineer the bandgap by controlling energy barrier height. In contrast, strained epitaxial growth techniques16 enable the in situ vapor phase deposition of both the dot material and the barrier materials, allowing one to engineer the QD energy levels not only by confinement size, but also by choosing the barrier energy height and carrier effective masses. However, the material systems suitable for this growth mode are limited due to the strict requirements on lattice parameter mismatch and crystal structure of both materials. Additionally, epitaxial growth modes, such as molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) are high-temperature or highvacuum processes, which adds to the cost of these techniques. Received: August 24, 2010 Revised: November 7, 2010 Published: February 14, 2011 934

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Figure 1. Bright-field TEM images at low and high magnification of ALD PbS films deposited directly onto SiO2 membrane TEM grids as a function of number of ALD cycles. (Bottom right) Plot of grain size extracted from TEM images.

ALD provides an alternate technique for fabrication of quantum confined structures. ALD is a modified version of MOCVD in which the chemical reaction is broken into two half-cycles, which saturate the surface due to the self-limiting chemistry of the precursor molecules.17 This allows for subnanometer precision in film thickness, as thickness is controlled by the number of ALD cycles rather than temperature or time. Here we exploit the substrate-inhibited growth mode18 observed in the initial cycles of certain ALD chemistries, leading to nucleation and growth of islands rather than formation of a dense film. This suggests a route to directly deposit QDs that are not capped by organic groups or restricted by epitaxial requirements. The quantum mechanical barrier can be easily defined by choice of a capping layer deposited by ALD. Additionally, formation of high-aspectratio coatings is possible with ALD,19 allowing for uniform coverage of 3D nanostructured architectures. Recently, our group has demonstrated quantum confinement effects in lead sulfide (PbS) thin films deposited by ALD,20 suggesting its potential for QD deposition. ALD PbS films were deposited with a varying number of cycles on SiO2 membrane transmission electron microscopy (TEM) grids (Ted Pella, Inc.) to study the nucleation and growth behavior of the films. The depositions were performed in a customized flow-type reactor designed for H2S compatibility.21 Note that H2S is a highly toxic, flammable, and corrosive gas, and

special care should be taken when performing experiments, as described in ref 21. The precursors used were bis(2,2,6,6-tetramethyl-3,5-heptanedionato)lead(II) (Pb(tmhd)2), (Strem Chemicals, Inc.) and a gas mixture of 3.5% H2S in N2. The Pb(tmhd)2 precursor was sublimated at 140 °C. The substrate temperature during all ALD depositions was 160 °C. Argon was used as a carrier gas with a flow rate of 10 sccm. Plane-view bright-field TEM images and in situ HRTEM video (10 frames/s) were taken by an FEI Tecnai G2 F20 X-TWIN operated at an accelerating voltage of 200 kV. Plane-view images were taken of the films after 10 to 80 ALD cycles. The results are shown at two magnifications in Figure 1. Several trends can be seen in the TEM images. The first observation is that polycrystalline PbS films initially nucleate as nanoscale crystallites on the SiO2 surface. The initial crystallites formed have diameters below the Bohr exciton radius of PbS (∼18 nm),22 suggesting the possibility of depositing PbS QDs in the early stages of ALD growth. The dimension of the crystallites normal to the surface is determined primarily by the number of ALD cycles, although the shape of the islands changes as crystallization occurs. These crystallites appear dark in the TEM image primarily through absorption contrast, as the density of PbS (7.61 g/cm3) is significantly larger than that of SiO2 (2.20 g/cm3). The nucleation site size and density is seen to increase with the number of cycles. These nucleation sites then evolve into 935

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Figure 2. (a) Evolution of an ALD PbS film during e-beam irradiation as a function of exposure time. Yellow arrows show coalescence of smaller dots into larger dots, green arrows show sublimation of dots. (b) in situ observation of coalescence of two dots into one. A bridge forms with aligned (200) planes between the dots.

energetically favorable reaction with -OH groups on the SiO2 surface.23 This reaction can be described as

crystalline grains, which merge together to form a polycrystalline film. Additionally, the shape of the grain changes from roughly circular to rectangular with increasing cycle number, which is consistent with the formation of cubic, rock-salt crystallites when there is sufficient material present on the substrate. Selective area diffraction (SAD) patterns confirm that the grains are randomly oriented with the rock-salt structure (see Supporting Information). Because of the ability of ALD to conformally coat high-aspect-ratio gaps, it is likely that the images show crystallites that were deposited on both the front and backside of the grids. Therefore, the uncertainty in the apparent density of nucleation sites in these images is within a factor of 2. To quantify the growth behavior of ALD PbS films, the average crystallite size is plotted in Figure 1. We approximated to a square shape of grains, and the unit of grain size used is the average edge length of a square, based on the area of a dot (see Supporting Information). The standard deviation in crystallite size is seen to increase with cycle number, indicating that certain grain orientations may grow faster than others, and nucleation of new sites occurs along with grain growth. To understand the initial formation of nucleation sites on the SiO2 surface, several chemical phenomena should be considered. The first is the reactivity of the initial ALD cycle with the surface. The initial cycle consists of a pulse of Pb(tmhd)2, which has an

Si - OH þ PbðtmhdÞ2 f Si - O - Pb - tmhd þ H - tmhd ð1Þ This reaction can be limited by several factors, including the availability of reactive sites on the SiO2 surface. Surface coverage of hydroxyl groups on SiO2 is not 100% and decreases with increasing temperature above 100 °C.24 Therefore, the reactivity of the substrate surface with the precursor will not be complete during the ALD reaction. Furthermore, there will be a steric hindrance that acts to prevent the formation of a complete monolayer per precursor pulse due to the relatively large size of the β-diketonate ligand, which will limit the spacing between chemisorbed Pb atoms during a saturated ALD pulse. Therefore, after the first precursor pulse, there will be a scattering of Pb atoms on the surface based on limited reactivity of the SiO2 surface and the Pb(tmhd)2 precursor. During the subsequent H2S pulse, a second ligand-exchange reaction occurs, in which the remaining tmhd ligands on the surface are removed to the gas phase, and replaced with -SH groups bonded to the Pb atoms. This can be described as Si - O - Pb - tmhd þ H2 S f Si - O - Pb - SH þ H - tmhd

ð2Þ 936

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At this point, there will be pairs of Pb-S groups dispersed over the surface with a spacing defined by the separation of Pb atoms from the initial precursor pulse. Since this separation is significantly larger than the Pb-Pb separation in crystalline PbS, a crystalline phase is not formed during this initial AB reaction cycle. After several ALD cycles, crystalline grains of PbS will form with the interatomic spacing defined by the lattice parameter. For a crystalline phase to form, some surface diffusion is likely to occur. This surface diffusion will involve breaking of chemical bonds and formation of new bonds based on the PbS crystal structure. One possibility is that this crystallization begins after the first AB reaction cycle, by diffusion of either individual atoms or Pb-S pairs. These atoms then bond with neighboring Pb-S pairs to form PbS clusters. These clusters can then act as nucleation sites for further growth of PbS crystallites, if the reactivity of ALD PbS with Pb-SH surface clusters is greater than with the substrate surface. Alternately, growth of randomly oriented grains may occur during subsequent cycles, and grains with preferential orientations may dominate the early stages of growth, leading to the formation of islands. Furthermore, owing to the relatively low temperatures of the ALD reaction, the rate of crystallization will likely be limited, suggesting the ability to further modify the film morphology by subsequent annealing. Interestingly, electron-beam irradiation of the PbS crystallites in the TEM causes an evolution of the film morphology with exposure time. While the as-deposited crystallites quickly merge into grains with increasing ALD cycles, e-beam exposure leads to the formation of isolated nanoparticles with sub-10 nm diameters on the surface. The evolution of a PbS film deposited with 10 cycles is shown as a function of e-beam exposure time in a series of high-resolution (HR)TEM images in Figure 2a. We speculate that the irradiation effect on the PbS morphology is due to localized specimen heating during e-beam exposure. There could be a large temperature increase due to absorption and inelastic scattering of electrons in the film and SiO2 membrane, which is aided by limited heat transfer to the surroundings due to the low thermal conductivity of SiO2. This substrate heating by e-beam irradiation has been previously observed experimentally.25 To estimate the temperature of the substrate during e-beam exposure, the lattice parameter of a PbS nanoparticle was measured using in situ HRTEM as a function of irradiation time. The lattice parameter was observed to increase with irradiation time, suggesting that thermal expansion is occurring. By extrapolating reference data on the lattice parameter of PbS as a function of temperature, the nanocrystal temperature was estimated to increase from 300 to 850 K during an irradiation period of 180 s (see Supporting Information). Additionally, the electron beam could ionize the material and break chemical bonds,26 allowing for easier surface diffusion to occur. Coalescence of the PbS material into distinctive nanoparticles can be seen after exposure to around 10 min of e-beam irradiation as indicated in yellow in Figure 2a. These nanoparticles have diameters within the strong confinement regime for PbS, indicating that they can behave as QDs. This coalescence of asdeposited PbS islands must be accompanied by surface diffusion, which has also been directly observed as migration of PbS crystallites on the surface during e-beam irradiation. This leads to an increase in the size of the nanoparticles due to the thermodynamic driving force toward obtaining a lower surface-tovolume ratio.27 This surface diffusion and coalescence behavior

Figure 3. Low- and high-magnification TEM images of PbS QDs after 10 ALD cycles. (a) As-deposited (b) after 11 min of e-beam irradiation. Average line profiles of dark contrast level (a-3 and b-3) are extracted from the dashedyellow rectangular regions. (c) A shape transformation from flat islands to dome-shaped particles after irradiation is seen as a sharp increase in contrast.

under e-beam irradiation has been observed in other systems, including gold nanoparticles.28,29 Concurrently, sublimation and shrinkage of PbS nanoparticles after longer exposure times is indicated in green in Figure 2a. The TEM chamber pressure (8.8  10-8 Torr) is likely to be lower than the pressure at the triple point of PbS,40 allowing for the film to sublime at a reduced temperature compared to higher pressure conditions. Furthermore, the melting or sublimation point of materials has been shown to decrease in particles with nanoscale dimensions.13,30-32 These two competing behaviors (enlargement by coalescence and shrinkage by sublimation) can be used to control particle size, since coalescence occurs at a much faster rate than sublimation; the average size of the QDs increases during shorter exposure times due to coalescence and shrinks with increasing time as sublimation begins to dominate. Furthermore, the standard deviation in particle size is seen to initially increase due to random coalescence and eventually decreases during sublimation (see Supporting Information). This suggests that annealing can be used to modify both the average particle size and reduce the variation in dot size as deposited by ALD. 937

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Figure 4. TEM images of SiNWs coated by ALD PbS shown at different magnifications after (a) 10 cycles, (b) 20 cycles, and (c) 40 cycles. Also shown is the effect of 20 min e-beam irradiation on the dot-coated SiNWs.

contrast is relatively flat across the dot before irradiation, while the slope of the profile increases dramatically after irradiation, which is in agreement with a dome-shaped thickness profile. These results suggest a new method for fabrication of PbS QDs through the nucleation of crystallites during the initial stages of ALD growth. The size, shape, and density of QDs can be controlled by a number of variables, including number of cycles, reactivity between the precursor and substrate, and subsequent e-beam annealing. While e-beam irradiation in a TEM is not a practical large-scale manufacturing technique, it allows for the unique in situ observation of the evolution of the film morphology resulting from irradiation. It can also be used as a technique to precisely control the position or size of QDs in nanoscale experiments. This could in principle be replaced with other forms of annealing or irradiation, including larger scale photon, ion, or electron exposure. To demonstrate the real advantage of ALD for fabrication of QD arrays and the feasibility of coverage of high-aspect-ratio nanostructures with PbS QDs, ALD PbS was deposited on the surface of SiNWs grown by vapor-liquid-solid (VLS) growth.36,37 SiNWs were grown on silicon wafers and on stainless steel 304 mesh. For the silicon substrates, Au nanoparticle catalysts were deposited by functionalizing the substrates with 0.1% w/v aqueous poly-L-lysine solution and drop-casting the nanoparticles onto the substrates. For the stainless steel mesh, the catalyst consisted of a 25 nm Au film deposited by thermal evaporation. The substrates were preheated in a tube furnace at 490 °C for 10 min under vacuum after which a 50 sccm flow of silane (2% SiH4 in Ar) was introduced for 12-15 min at 490 °C

Additionally, PbS nanoparticles experience a strong interaction during coalescence. While the driving force for an increase in particle diameter is thermodynamic, we observed that neighboring nanoparticles tend to move directly toward each other, rather than experiencing random diffusion on the surface. Furthermore, their crystal orientation is synchronized with a bridge of aligned (200) planes between the particles. This is shown in a series of snapshots taken during an in situ HRTEM video in Figure 2b (full video file available in Supporting Information). To facilitate this bridging behavior, atomic rearrangement was observed in the nanoparticles during irradiation (see Supporting Information text), which has been previously observed during local heating of nanoparticles by an electron beam.33 Another phenomenon that is observed during in situ irradiation is a shape transformation from flat islands to more domeshaped QDs. As-deposited crystallites during the ALD process provide low contrast in the TEM image, which is a result of a relatively flat “disk” shape. However, after irradiation, the contrast of the nanoparticles is increased dramatically relative to the surroundings. This is shown after 11 min of e-beam irradiation in Figure 3 by bright-field absorption contrast. As a result of the e-beam annealing process, the crystallite morphology changes to a dome shape to lower the surface to volume ratio,34,35 which is thermodynamically favorable due to the Gibbs-Thomson effect. This in turn provides more contrast in the TEM image due to a greater thickness variation between the center of the QD and the substrate. To quantify this shape transformation, average line profiles of dark contrast were measured before and after irradiation, as shown in Figure 3a-3,b-3. The profiles show that the 938

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photodiode array detector. Measurements were corrected for the spectral response of the system using a blackbody emitter at 3100 K. The samples were measured as-deposited by ALD without any annealing. The results are shown in Figure 5. The bare SiNWs demonstrate broadband near-infrared PL with relatively low intensity, which is typical of unannealed VLSgrown SiNWs,38 and are presumably due to a relatively large number of subgap defect states in the VLS-deposited material.39 On the other hand, after only 10 cycles of ALD PbS the PL intensity increases dramatically and has an approximately Gaussian distribution of wavelengths. This is consistent with an ensemble of QDs of different sizes, as seen in the TEM images. While this may be a disadvantage for applications such as lasers, where a single wavelength is desired, other applications such as solar cells may benefit from a broad distribution of wavelengths available for excitonic transitions. When the number of cycles was increased to 30, a clear shift in the PL distribution to longer wavelengths in the infrared can be seen, presumably due to the increased size of QDs. While the InGaAs detector used in these experiments cuts off at approximately 1600 nm in the IR, the trend of increased wavelength in emission is clearly demonstrated in these samples. This is indicative of quantum confinement in the PbS QDs, illustrating that their optical properties can be controlled simply by varying the number of ALD cycles. The ability to conformally coat high-aspect ratio surfaces with a single layer of QDs whose size can be controlled by varying the number of deposition cycles is a unique and powerful ability. This suggests the possibility of developing a number of novel device architectures that benefit from the multitude of advantages of nanostructured materials, including improved absorption in QD solar cells. Additionally, this method could be applied to other applications, including sensors, LEDs, and photocatalysis.

Figure 5. Photoluminescence spectra for bare SiNWs (blue), 10 cycles of ALD PbS (black), and 30 cycles of ALD PbS (red).

’ ASSOCIATED CONTENT

bS

Supporting Information. Structural analysis of as-deposited PbS QDs by ALD, size measurements of QDs, temperature of QDs during in situ e-beam irradiation, crystalline bridging and reorientation of QDs during e-beam irradiation. HRTEM video of QDs merging during e-beam irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 1-773-497-9532. Fax: 1-650-723-5034. Author Contributions

)

while the pressure of the system was maintained at 40 Torr. Figure 4 shows a series of TEM images of SiNWs coated by ALD PbS after various numbers of cycles, which were transferred to a TEM grid for analysis. It can be seen that the nucleated PbS nanocrystals are uniformly distributed along the SiNWs over large aspect ratios. Additionally, the nanocrystal size and density can be controlled by the number of ALD cycles. It is difficult to accurately assess the absolute value of particle density due to the curved surface of the nanowires and the fact that we observe QDs on both sides of the nanowire in the TEM image. However, a trend of increased size and density with increasing cycle number is evident. The curved surface of the nanowire also distorts an interpretation of the exact size of the 3D dot, which is projected onto a 2D image. If the PbS acquires a curved dome shape to conform to the surface of the nanowire, the apparent thickness of the particles will increase in the 2D image. Interestingly, the as-deposited nanocrystals appear to be more isolated and dome-shaped on the nanowire surface than the plane-view samples would suggest. We hypothesize that this is due to a higher surface diffusion rate on the nanowire surface compared to the flat SiO2 TEM grid, allowing dots to approach their thermodynamically favorable shape at the deposition temperature. There could also be differences in the surface properties of the 40 nm thick SiO2 grids and the thin native oxide on the SiNWs. The degree of this isolated dome-shaped geometry is shown to increase after 20 min of e-beam irradiation in Figure 4, column 4. To demonstrate the ability to modify the optical properties of the nanowires from quantum confinement effects in ALD PbS QDs, photoluminescence (PL) measurements were performed on bare SiNWs grown on stainless steel mesh substrates, and compared to QD-coated nanowires. Absorption measurements were also performed using UV-vis spectrophotometry. However, the signal-to-noise ratio was much higher in the PL measurements due to the small level of absorption in the ultrathin films deposited in this study and the significantly higher sensitivity of the instrumentation for collecting PL. Continuous-wave optical excitation was provided by a frequency-doubled Nd:VO4 laser at 532 nm, focused on the specimens with a 10 microscope objective to yield a pump intensity of ∼50 kW/cm2. The PL was collected by the objective, passed through a 750 nm longpass filter to reject the pump light, and focused onto the input slit of a spectrograph equipped with a liquid-nitrogen-cooled InGaAs

These authors contributed equally to this work.

’ ACKNOWLEDGMENT The authors thank Swagelok Co., Sunnyvale, CA, for providing H2S compatible ALD valves and advice. H.J.J. was supported as part of the Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001060. M.T.M. acknowledges support from the NDSEG Fellowship Program and a Stanford Graduate Fellowship. 939

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