Chemically Tunable Full Spectrum Optical Properties of 2D Silicon

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Chemically Tunable Full Spectrum Optical Properties of 2D Silicon Telluride Nanoplates Mengjing Wang, Gabriella Lahti, David Williams, and Kristie J. Koski ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02789 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 10, 2018

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Chemically Tunable Full Spectrum Optical Properties of 2D Silicon Telluride Nanoplates Mengjing Wang†‡, Gabriella Lahti†, David Williams†, and Kristie J. Koski†* †

Department of Chemistry, University of California Davis, Davis California 95616, USA

‡

Department of Chemistry, Brown University, Providence, Rhode Island, 02912, USA

*[email protected]

KEYWORDS: silicon telluride, Si2Te3, zero-valent intercalation, 2D materials, layered materials, germanium doping

ABSTRACT: Silicon telluride (Si2Te3) is a two-dimensional, layered, p-type semiconductor that shows broad near-infrared photoluminescence. We show how, through various means of chemical modification, Si2Te3 can have its optoelectronic properties modified in several independent ways without fundamentally altering the host crystalline lattice. Substitutional doping with Ge strongly redshifts the photoluminescence while substantially lowering the direct and indirect band gaps and altering the optical phonon modes. Intercalation with Ge introduces a sharp 4.3 eV ultraviolet resonance and shifts the bulk plasmon even while leaving the infrared response and band gaps virtually unchanged. Intercalation with copper strengthens the photoluminescence without altering its spectral shape. Thus silicon telluride is shown to be a

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chemically tunable platform of full spectrum optical properties promising for opto-electronic applications.

Silicon telluride (Si2Te3) is a red, transparent, p-type layered semiconductor. It has an indirect gap near 1 eV and direct gap near 2 eV.1-4 It typically possesses a large defect density of up to 1017 cm-3 which gives rise to broad photoluminescence in the near infrared range around 640-850 nm.5-8 Silicon telluride offers potential in application given its processing compatibility with silicon.6 Recent studies suggest that silicon telluride may be an exceptional thermoelectric possessing excellent electrical conductivity, low thermal conductivity, and a remarkably high ZT value of 1.86 at 1000K.7 Other investigations reveal it as a precursor to accessing topological quantum phases of matter through pressure-driven phase transitions.9,10 Silicon telluride can be synthesized through traditional solid state methods including iodine transport or bulk solid state growth.1,11 Exposure of a silicon substrate to a tellurium atmosphere also generates polycrystalline Si2Te3.12 Recently, micro-sized and nano-sized silicon telluride with lower defect density and controllable morphologies of nanoribbons and nanoplates were shown to be accessible through a large area vapor-liquid-solid mechanism and vapor phase growth techniques.6

Given the broad, defect-associated photoluminescence (PL) of silicon telluride in the infrared range,6,8 a promising argument can be made for it as an infrared-detector material. An example of a traditional infrared-detector material is mercury cadmium telluride with a tunable bandgap produced by altering mercury and cadmium concentrations.13-15 Issues with current Hg-Cd-Te technology include the relative toxicity of the metals as well as a need for cooling. Accessing a tunable band structure with silicon telluride may offer a promising material solution if a similarly 2 ACS Paragon Plus Environment

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effective tunability as that of the mercury-cadmium-tellurium system can be achieved. We demonstrate that the opto-electronic properties of silicon telluride can be tuned through two approaches. First, germanium doping performed in growth is shown to achieve dopant-tunable photoluminescence along with a tunable electronic band-structure. The direct gap can be wellcontrolled with substitutional doping to provide both a tunable indirect bandgap and direct bandgap from 2.11 eV to 1.79 eV. Second, intercalation of zero-valent elements (Cu and Ge) into the van der Waals gap of Si2Te3 is shown to alter the electronic bandgap, valence interband transition region, plasmonic properties, and potentially conductivity, while leaving the shape of the broad photoluminescence peak unchanged. Intercalation of copper leads to enhancement of the photoluminescence. Coincident with chemical tuning are changes to the optical phonons as investigated with Raman spectroscopy. These results show promise for the chemical tunability of silicon telluride nanoplates across the full optical spectrum. Evidence of the simplicity of synthesis, reduced toxicity, and the ease of controlling the material behavior offers promise for future silicon-based applications.

RESULTS AND DISCUSSION Si2Te3 is a layered chalcogenide (Fig. 1a), with space group ܲ − 31ܿ. It has a trigonal crystal structure with lattice constants a = 7.429 Å and c = 13.471 Å and is arranged in 2D layers with van der Waals gaps between layers. Each layer consists of a bilayer of close-packed Te atoms with Si dumbbells filling the interlayers in 2/3 of the octahedral sites, leaving 1/3 of the octahedral sites vacant.16 Occupancy of silicon sites is variable, as are the orientations of the Si dumbbells, and this variability of structure is in part responsible for the material’s strong, broad infrared PL response and variable bandgap.3-5 Si2Te3 substitutionally doped with controlled


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amounts of germanium was synthesized through vapor phase growth as shown in the growth schematic in Fig. 1b and outlined in the materials and methods. Growth of these plates is entirely performed in vapor phase as opposed to large area vapor-liquid-solid growth, which leads to a mix of nanoplates situated upright on the growth substrate and plates lying flat on the substrate.6 Nanoplates are grown from a source powder of silicon, tellurium, and germanium, and have lateral sizes on the order of 10-20 microns and thicknesses on the order of 40 nm to 150 nm (Supporting Information; Fig S3). Nanoplates can be exfoliated mechanically to form thinner plates. Stoichiometric amounts of silicon and germanium in grown nanoplates are controlled through the concentration of germanium powder added to the precursor (Fig. 1b). Ge dopant concentration is found to be linear, within error, with respect to the precursor germanium source concentration (Fig. 1c). Nominal germanium doping saturates near x = 0.22 (Si1.78Ge0.22Te3), beyond 0.22 Ge, phase separation occurs from competing formation of germanium telluride and Ge0.9Si0.1Te which has been noted in bulk growth (see Fig. S2; Supporting Information).17 Optical images of Si2-xGexTe3 flat plates shows variation in color that corresponds to the added germanium (Fig. 1d). Nanoplates become a deeper, purple-red color with addition of germanium yet retain their original transparency. Vapor-phase grown Si2Te3 nanoplates are low-defect and only show a few Raman modes (Fig. 1e) with two very strong low-frequency modes and weaker high frequency modes that can be compared to bulk-grown Si2Te3.18 Vibrational modes of silicon telluride nanoplates were confirmed using polarization studies (Supporting information; Fig S5). The first peak at 99 cm-1 corresponds to an in-plane Eg mode. Measurements of bulk silicon telluride by Zwick et al,18 identified the low-frequency vibrational mode at ~120 cm-1 as an Eg mode. In flat nanoplates, a


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very strong mode appears at 127 cm-1 which can be assigned as an A1g mode through polarization investigations (Supporting information; Fig S5). The most intense peaks at 143 cm-1 can be assigned to an out-of-plane A1g mode. Broader stretches found around 500 cm-1, 310 cm1

, and 210 cm-1 belong to the in-plane Eg modes.18,19 These modes become more intense as a

sample becomes more defected through hydrolysis in air and loss of tellurium. It has been suggested that some of these modes may result from reordering of the silicon dumbbells into other possible sites leading to relaxation of conservation of momentum rules.18,19 Nanoplate samples grown through the vapor-phase route tend to have less defects than bulk material and rarely show modes higher than 200 cm-1, or else those modes possess low, almost undetectable intensities. Few to no peaks are seen higher than 200 cm-1 in germanium doped silicon telluride nanoplates, which indicates a low presence of defects. Also, no shift of the Raman modes is seen with germanium doping within error. Detailed Raman shifts are listed in Table S1 in Supporting Information. Doping with germanium enhances the intensity of the low-frequency 127 cm-1 A11g mode relative to the 143 cm-1 A21g mode (Fig. 1e; inset). Germanium and silicon are similar sizes but germanium is a heavier atom. Substitution of Ge into the host can increase the interplanar oscillator strength and changes the effective mass resulting in intensity enhancements by replacing the planar neighboring atoms. Silicon telluride has a broad photoluminescence peak from around 700 to 900 nm. Previous measurements of photocurrent in bulk Si2Te3 attribute the PL to a deep trap state.5,8 Doping with germanium in Si2-xGexTe3 causes the photoluminescence to sharpen and results in a PL peak redshift (Fig. 1f). Specifically, the curves show little change on the low-energy side but immediately lose a major contribution peaking near 750 nm (1.65 eV) even at the lowest concentrations tested. Increasing concentrations of Ge dopant do not alter the PL further. Doping


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with germanium clearly affects the trap emission behavior but that impact to the PL occurs at the lowest substitutional concentration. The redshift of photoluminescence peak can be attributed to doping of germanium to lower the direct band gap energy as seen from Fig 6c. This is expected considering the difference in bandgap of the pure materials of silicon (1.1 eV) and germanium (0.7eV). Pure germanium has a lower bandgap and can be used in Si/Ge alloys to reduce the effective bandgap.20,21 Here, this is performed with substitutional doping of germanium in silicon telluride with the same effect. Zero-valent atomic species can be intercalated into the van der Waals gaps of layered silicon telluride to further alter the optical properties (Fig. 2a). Zero-valent intercalation proceeds by creation of a dilute zero-valent element in solution.22,23 The metal intercalates into guest sites commensurate or incommensurate with the host. The metal can alter the semiconducting behavior of the material from n-type to p-type,24 add interband states,25,26 or alter the conductivity of the material.27 Since the intercalant is zero-valent, it tends not to bond strongly with the host itself thus it does not alter the overall structure of the host material. Zero-valent copper and germanium were intercalated into silicon telluride nanoplates using a disproportionation redox reaction of Cu+1 to Cu0 in acetone or germanium (II) bromide to Ge0 at 150oC in octadecene, respectively (see Materials and Methods). Intercalation with a metal shows only minor optical changes within the nanoplate as demonstrated through optical images (Fig. 2b-e). Nanoplates appear somewhat brighter with intercalation but do not show the purple enhancement seen through Ge-doping. Optical brightness may be due to an increase of the photoluminescence yield or an enhancement in the transparency of the nanoplate with intercalation.27 Intercalation of a metal like copper is known to increase the numbers of defects in a host 2D layered material.22,23 Copper is small and intercalates at the edges of a host as well as


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diffuses through the layers of the crystal lattice itself resulting in defects to the host.22 These defects are reflected in the increase in the Raman modes above 300 cm-1 which were previously attributed to disorder-induced density-of-states effects.18 The low-frequency mode at 127 cm-1 characteristic of the nanoplates, completely vanishes from copper intercalated silicon telluride. Conversely, Raman spectra of germanium-intercalated silicon telluride shows an enhancement of that low-frequency A11g mode relative to the A21g mode. The relative change of the intensity of the Raman modes is shown in the inset of Fig. 2c. The photoluminescence spectra of germanium- and copper-intercalated Si2Te3 (Fig. 2d) show no shift or change from the broad PL of unintercalated Si2Te3, providing further evidence that intercalation does not substantially alter the host. Since the photoluminescence results from deep trap states and the host remains unchanged, the PL remains broad. Because copper creates more defects within the host, the overall quantum yield of the photoluminescence increases as more trap states may be created through copper intercalation (Supporting information; Fig. S7). Germanium intercalation results in a decrease in quantum yield of the PL possibly because of the high temperatures required for intercalation which may anneal out some defects of the Si2Te3 host. Intercalation of a metal can make a material more conductive which means that intercalation can be used to increase the conductivity of Si2Te3 without quenching the desirably broad photoluminescence properties.27 X-ray diffraction (XRD) of Si2-xGexTe3 is shown in Fig. 3a as a function of Ge-dopant, x. All diffraction patterns in Fig. 3 are normalized to the (008) peak. Diffraction patterns show intensity variation due to texturing of the powder sample. XRD of Si2-xGexTe3 shows a change in the overall lattice constants from a = 7.423±0.003 Å and c = 13.49±0.01 Å to a = 7.438 ± 0.01 Å and c = 13.47 ± 0.01 Å with doping. The change in unit cell volume (Fig. 3; inset) reflects this increase. XRD of doped samples shows little change from the host Si2Te3 with the exception of


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some texturing. Given the small size of the nanocrystals, it is not possible to precisely, determine the germanium positions, however we know from composition measurements that the germanium is largely substituting for silicon, likely in the form of Ge-Si and Ge-Ge dumbbells. As the former should be more prevalent at low Ge concentrations and the latter at high concentrations, and as the effects of Ge substitution on the optical properties saturates quickly at low concentrations, this suggests that the altered optical properties are primarily due to Ge-Si dumbbells, as the observed physical properties manifest and cease at low germanium concentrations where Si-Ge dumbbells would be dominant. XRD of Cu and Ge intercalated Si2Te3 is shown in Fig. 3b. Zero-valent intercalation can lead to an expansion or a contraction of the host lattice depending upon the amount of metal intercalated and the nature of the intercalant within the host.23,26 XRD shows an expansion of the host unit cell volume from 643.6 ±0.8 Å3 to 645.0±0.3 Å3 with Cu-intercalation at 3.5 atm % and a contraction of the host from 643.6±0.8 Å3 to 642.3±0.6 Å3 with Ge intercalation at 6.5 atm %. No obvious superlattice peaks, which are characteristic of high density zero-valent intercalation, are seen in the XRD, but this is typical for intercalant concentrations below 10 atomic percent, such that for similar materials the superlattice spots may be visible in electron diffraction but are too weak to be easily seen in XRD.22,23,26 XRD of Ge-intercalated Si2Te3 (Ge0.35Si2Te3; Fig 3b) shows similar texturing to Ge-doped Si2Te3. XRD of Cu-intercalated Si2Te3 (Cu0.18Si2Te3; Fig. 3b) shows strong enhancement of multiple peaks, most notably (112) relative to the (008) peak with intercalation. This may be due to changes of crystallographic texture or it may be indicative of disorder in the plane-normal direction due to stacking faults, which are known to result from zero-valent Cu intercalation in similar materials.22


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Energy dispersive X-ray (EDX) elemental mapping was used to determine the elemental composition and to show that intercalated and Ge-doped Si2Te3 nanoplates have a homogeneous distribution of added species throughout the entire sample. Fig. 4a shows an example elemental map (EDX spectra; Fig. 4b) of a silicon telluride nanoplate doped with germanium at Si1.88Ge0.12Te3. Germanium is uniformly distributed throughout the entire plate. A 1D slice through the plate (Fig. 4c) shows that germanium dopant concentration is also equal throughout the center and sides of the nanoplate. Selected area electron diffraction (SAED) patterns show that germanium doping maintains the trigonal crystal structure of silicon telluride (Fig. 4d-e). Intercalation with a zero-valent element also distributes uniformly throughout the platelet. Fig. 5a shows an example of silicon telluride intercalated with 3 atomic percent of copper (Cu0.15Si2Te3). Copper is uniform throughout the entire plate. A maximum intercalant percentage of 6.48 atomic percent of germanium (Ge0.34Si2Te3; Fig. 5b) and a maximum intercalant percentage of 3.47 atomic percent of copper (Cu0.18Si2Te3) can be achieved. SAED of Ge- and Cu-intercalated Si2Te3 shows the appearance of diffuse lines and diffuse near the centers of Bragg-spot triangles, offering further evidence of intercalation-induced stacking faults and possibly intercalant ordering within the host.23,25 Using doping or intercalation to chemically alter the opto-electronic properties of Si2Te3 is most strongly revealed in UV-VIS-NIR absorbance data (Fig. 6a-d). Silicon telluride has a direct bandgap at around 2.0 eV and an indirect gap at around 1 eV. A Tauc plot analysis was used to determine the bandgap energies (Eg) from absorption data. Doping with germanium Si2-xGexTe3 shows a tunable direct gap from 2.11 eV to 1.79 eV and tunable indirect gap from 0.98 eV to 0.72 eV. This is a significant change in the electronic bandgap suggesting that the substitutional germanium doping considerably alters the band structure of silicon telluride. For the most part,


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silicon telluride has flat bands,4,9,10 and the germanium dopant likely lowers the conduction band energies at the gamma point resulting in a decreased direct bandgap. This is similar to the consequence of using germanium doping in silicon to engineer the electronic bandgap while also suppressing defects. Germanium has a larger lattice constant and has a slightly larger atomic size than silicon; it will introduce a strain when doped directly into the lattice.20,21 As a result, the effective bandgap will decrease. Intercalation can introduce interband states and/or can distort the host lattice through electronic interaction. The net result will be a shrinkage of the bandgap either from the interband states or the distorted lattice. Intercalation of copper (Fig 6d) may slightly decrease both the direct and indirect band gap energies, though the error bars overlap so this conclusion is tentative. Similarly, Ge intercalation may have decreased the direct gap or it may have had no effect, but it rendered the signature of the indirect gap so weak that it could not be detected. These measurements demonstrate that the electronic properties and bandgap energies can be tailored chemically using doping and possibly using intercalation while also modifying the photoluminescence properties. Electron energy loss spectra (Fig. 6e,f) of silicon telluride shows a broad bulk plasmon at around 15.5 eV. Gradual onset edges at 40 eV from the Te N4,5-edge (Fig. 6e,f) and at 29 eV from the Ge M4,5-edge are seen in spectra. Doping with germanium (Fig. 6e) shows little or no change in the bulk plasmon. This is unsurprising because there is almost no change in the total valence electron density, as we are substituting one group IV element with another and the unit cell volume change is small. With minor amounts of copper intercalation at 1.4 atm %, little or no shift is seen in the bulk plasmon. With intercalation of germanium at ~1.1 atm %, the bulk plasmon shifts significantly to 16.9 eV, while a new peak appears at 4.3 eV. Both results are


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unexpected. The large plasmon shift cannot be interpreted as merely due to an increase in valence electron density, since it represents a 9% increase in bulk plasmon energy which would imply a 19% increase in valence electron density, which is far larger than the Ge intercalant density could account for. These peak positions are also not strongly dependent on the Ge intercalant concentration. Thus it appears that the Ge intercalant introduces a new, sharp, ultraviolet resonance, altering the valence band structure sufficiently to substantially shift the bulk plasmon itself even at rather low concentrations. The 4.3 eV peak is reminiscent of peaks in analogous 2D layered systems such as graphite or intercalated Bi2Se3 where the additional peak can be attributed either to an interband transition caused by a spike in the electronic density of states or to bulk and surface plasmons associated with collective excitations in a quasiindependent subband such as the electrons in graphite.29,30 Despite an apparent substantial modification of the electronic structure in the ultraviolet range, Ge intercalation had little to no effect on the infrared response such as shown in Fig. 2. Thus the two means—substitutional doping and intercalation—of introducing Ge into Si2Te3 have entirely distinct, independent effects on the optoelectronic properties, a promising result from the perspective of tuning the material properties for specific applications. The difference seen between intercalation and doping of silicon telluride can be explained by the electronic interaction of the intercalant or the dopant. The intercalant does not bond with the host but can introduce tunable intrabrand electronic states as has been seen in other systems.22,25 The germanium dopant, however, does bond with the silicon telluride host intimately altering the electronic band structure as it alters the nature of the silicon dumbbells of the crystal. CONCLUSIONS


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This work demonstrates that germanium doping and zero-valent intercalation of silicon telluride are effective routes to alter the electronic band structure, plasmonic properties, and photoluminescence response, with markedly distinct effects associated with intercalation vs. substitutional doping. Specifically, Ge substitution had substantial effects on the infrared optical properties including the direct and indirect band gaps and the photoluminescence, while it had negligible impact on ultraviolet properties such as bulk plasmon energies. Conversely, intercalation could introduce a sharp, new ultraviolet resonance, shift the bulk plasmon energy, and strengthen the photoluminescence even while having practically no effect on the band gaps or the shape of the photoluminescence spectrum. Bulk silicon telluride typically shows increased electrical resistivity with relatively large concentrations of impurities,5,11,28 and this may be detrimental for some optoelectronic applications. However, intercalation of zero-valent metals,27 like copper, may be an effective route to increase the electrical conductivity of silicon telluride. With further investigation into the mid- to far-IR range and density functional theory calculations, we believe these methods to chemically tune Si2Te3 may lead to significant adaptability of this nanomaterial in semiconductor and optoelectronic applications.

MATERIALS AND METHODS Preparation of Silicon Telluride and Germanium Doped Silicon Telluride. Silicon telluride (Si2Te3) was grown through the previously published vapor-phase growth procedures.6 Fused silica substrates were placed downstream at 12-15 cm from a ceramic boat in the center containing well-mixed 0.18 g Te and 0.07 g Si which was heated to 800oC at a ramp rate of 60oC per minute. Germanium doping was performed by adding stoichiometric amounts of germanium powders to the ceramic boat while keeping the concentration of silicon used constant. No Te was


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placed upstream and all growth was done using simple vapor phase mechanisms using a carrier gas of high purity argon for transport. After reaching 800oC, the furnace was slowly cooled to room temperature. Nanoplates were stored under inert N2 in a glovebox. Intercalation of Copper and Germanium. Zero-valent intercalation of copper and germanium was performed using disproportionation redox reactions of Cu(I) and Ge(II).22,23 Specifically, a growth substrate containing Si2Te3 nanoplateles was placed in a round bottom flask with Liebig condenser under N2 using a Schlenk line. For Cu intercalation, 5 ml of 10 mM copper(I) tetrakisacetonitrile hexafluorophosphate in anhydrous acetone was prepared in the glovebox, injected into the round-bottom flask, and heated to just under reflux at 50oC under a constant N2 atmosphere for 3 hours. For Ge intercalation, 10 mM GeBr2 in 5 ml of 1-octadecene was prepared in the glovebox and then injected into a round bottom flask with a Liebig condenser under N2 from a Schlenk line. The solution was heated to 160oC for 3 hours after which the substrate was removed from the flask and rinsed with acetone to be stored under an inert N2 atmosphere in a glovebox. All chemicals used were from Sigma Aldrich. Characterization. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) data were acquired using an FEI Scios equipped with Oxford X-MaxN EDX detector at a voltage of 10kV. Powder X-ray diffraction (XRD) data was collected using a Bruker Eco Advance with copper source (Cu Kα1 = 1.5406 Å and Kα2 = 1.5444 Å). Optical images were acquired using a Leica ICC50E camera and microscope. Transmission electron microscope (TEM) images and electron diffraction (SAED) were collected on a JEOL 2500. Scanning transmission electron microscope images (STEM) and electron energy loss spectra (EELS) were collected on a JEOL 2100fac with a Gatan GIF. EELS spectra were collected from the center of the nanoplate. Additional EDX spectra were acquired on the JEOL 2100fac using


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an Oxford XMaxN-TSR EDX detector Photoluminescence data was acquired from the center of an isolated nanoplate with a Renishaw RM1000 using a 514.5 nm Ar+ laser for excitation at 0.025 mW. Raman spectra were collected from a home-built Raman system built around a Leica DMi8 inverted microscope, Semrock dichroic and razor edge filter, Princeton Instruments Isoplane SCT320, Princeton Instruments PIXIS CCD, and a 532 nm Coherent Sapphire SF laser with less than 0.2 mW focused on the sample through a 50X or 100X objective. UV-Vis-Nir absorption data was acquired from a PerkinElmer 950 with a 150 mm InGaAs integrating sphere. All measurements were collected at room temperature.

FIGURES


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Figure 1. Si2Te3 doped with Ge has a layered structure along (120) illustrated in (a), Si atoms with different transparencies representing different occupancies occupy 2/3 of the octahedral sites.6 (b) Hexagonal Si2-xGexTe3 plates were synthesized through vapor phase growth. (c) Ge content (x) is linear (red fit) with respect to the ratio of germanium to tellurium concentration used in growth below the nominal doping limit. (d) Optical images of Si2-xGexTe3 with varying Ge dopant concentration (x) show color variation. (e) Normalized Raman spectra with inset show the change in ratio of intensity of A11g to A21g as a function of dopant content x. (f) PL emission spectra are shown as a function of dopant content x in the inset.

Figure 2. (a) Zero-valent copper and germanium can be intercalated into Si2Te3 layers. (b) Optical images of Si2Te3 before and after intercalation show little color variation. (c) Raman spectra show no shift of phonon modes, but a change in intensity is noted with the inset. (d) PL peak emission does not shift with intercalation.


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Figure 3. (a) XRD of Si2Te3 doped with varying amounts of germanium show change in the unit cell volume as a function of doping content x (inset). (b) XRD of Si2Te3 intercalated with copper and germanium. Data is scaled to the (008) peak.


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Figure 4. (a) SEM-EDX mapping across a Si1.88Ge0.12Te3 nanoplate shows uniform distribution of Ge dopant, (b) Corresponding EDX spectrum. (c) A line profile (a; white line) shows the uniform distribution. (d) Electron diffraction of Si2Te3 and (e) Si1.85Ge0.15Te3 indicate the crystal structure is maintained with doping.


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Figure 5. (a) SEM-EDX map of Cu-intercalated Si2Te3 nanoplate shows a uniform distribution of copper. (b) Sample EDX spectra of Ge-intercalated Si2Te3 and (c) Cu-intercalated Si2Te3. Note the strong Cu and Ge signals. Electron diffraction of (d) Ge-Si2Te3 and (e) Cu-Si2Te3 show diffuse lines characteristic of intercalation (red arrow).

Figure 6. UV-VIS-NIR Tauc plot of (a) germanium-doped silicon telluride (Si2-xGexTe3) and (b) intercalated Si2Te3. Horizontal dashed lines represent the baseline. Direct bandgap (Eg,dir) and indirect bandgap (Eg,indir) of (c) germanium-doped silicon telluride and (d) intercalated Si2Te3 show tunability. (e) EELS spectra of Si1.85Ge0.15Te3 show no shift in bulk plasmon. (f) Intercalation with Cu shows a minor shift in the bulk plasmon (inset) while intercalation with Ge shows a new peak at 4.3 eV and a major shift in the bulk plasmon (inset).

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Supporting Information. Top-down view structure of silicon telluride; table of Raman shift of germanium-doped and intercalated silicon telluride; silicon telluride lattice constants after doping and intercalation determined through XRD; a nominal doping study; thickness analysis of germanium doped silicon telluride; high resolution TEM images of silicon telluride after doping and intercalation; Raman polarization investigations of Si2Te3; 1D Raman mapping of silicon telluride after germanium doping; normalized quantum yield of silicon telluride after germanium doping and intercalation; elemental mapping of silicon telluride after intercalation and doping; and absorption data of silicon telluride after doping and intercalation are provided. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] ACKNOWLEDGMENT This work was supported by the Office of Naval Research under grant ONR - N00014-16-13161. REFERENCES 1. Lambros, A. P.; Economou, N. A. Optical Properties of Silicon Ditelluride. Phys. Status Solidi B 1972, 57, 793-799. 2. Rau, J. W.; Kannewurf, C. R. Intrinsic Absorption and Photoconductivity in Single Crystal SiTe2. J. Phys. Chem. Solids 1966, 27, 1097-1101. 3. Ziegler, K.; Birkholz, U. Photoelectric Properties of Si2Te3 Single Crystals. Phys. Status Solidi A 1977, 39, 467-475.


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TABLE OF CONTENTS FIGURE:


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Chemically Tunable Full Spectrum Optical Properties of 2D Silicon

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