Synthesis and Material Properties of Bi2Se3 Nanostructures

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C: Physical Processes in Nanomaterials and Nanostructures 2

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Synthesis and Material Properties of BiSe Nanostructures Deposited by SILAR Rasin Ahmed, Yin Xu, Maria Gabriela Sales, Qiyuan Lin, Stephen John McDonnell, and Giovanni Zangari J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01692 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The Journal of Physical Chemistry

Synthesis and Material Properties of Bi2Se3 Nanostructures Deposited by SILAR Rasin Ahmed, Yin, Xu, Maria Gabriela Sales, Qiyuan Lin, Stephen McDonnell and Giovanni Zangari Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904.

Abstract Bi2Se3 was synthesized by a room temperature deposition technique, successive ionic layer adsorption and reaction (SILAR) method with the aim to understand the formation, crystallinity, optical properties and energy band structure of this material. The Bi2Se3 morphology was found to change from nanoparticles to nanocluster networks by increasing the SILAR deposition cycles. The crystalline structure of as-prepared Bi2Se3 determined from grazing incidence XRD (GI-XRD) pattern was found to have a mixed phase of metastable orthorhombic and rhombohedral phases which was further confirmed from our analysis of the Raman spectra. The optical bandgap of Bi2Se3 varied from 1.58 to 1.05 eV for 15 – 90 cycles of deposition, in contrast to the semi-metallic 0.3 eV bandgap exhibited by the pure rhombohedral phase. A schematic band diagram of Bi2Se3 prepared by 45 SILAR cycles was constructed for the mixed phase Bi2Se3. The flat-band potential was determined to be at 0.46 V vs. RHE from MottSchottky analysis. Low temperature annealing at 100°C for 1 hour resulted in the improvement of the rhombohedral phase fraction which was confirmed from analysis of GI-XRD pattern and pronounced E2g and A21g bulk vibrational modes in the Raman spectrum. The absorption cutoff after annealing was found to be red shifted combined with a sub-bandgap absorption above 0.78 eV. The results for post-annealing indicated the onset of an early stage transition from semiconductor to semi-metallic properties for Bi2Se3. Introduction The group V-VI compounds such as Bi2Se3, Bi2Te3 and Sb2Te3 have been traditionally investigated for thermoelectric conversion applications, and recently have also been discovered to behave as 3D topological insulators (TIs).1-4 These materials have a common rhombohedral crystal structure and a semi-metallic bandgap of 0.105 – 0.335 eV.5-6 In contrast, a second set of the same group V-VI compounds, Bi2S3, Sb2S3 and Sb2Se3 have all been reported to exhibit semiconducting properties with bandgaps of around 1.1 – 1.76 eV.7-9 This latter class of semiconductors have been shown to exhibit an orthorhombic crystal structure and have also demonstrated high radiation absorption coefficients making them suitable for visible light absorption in photonic devices and solar conversion technologies. Amongst these compounds, Bi2Se3 appears unique, since it has been reported to exhibit either a pure rhombohedral or a mixture of both rhombohedral and orthorhombic crystal structures, depending on the synthesis conditions.10-11 The most commonly reported crystal structure for Bi2Se3 is the rhombohedral phase (space group R3m), which exhibits a bulk bandgap of around 0.3 – 0.335 eV.4, 6 Formation of the alternative orthorhombic structure of Bi2Se3 has only been shown to occur using 1

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fabrication techniques operating at close to room temperature, while techniques that require high temperature or some form of annealing during growth only produce the rhombohedral phase. This orthorhombic structure (Pnma space group) has been reported to exist as a metastable phase under ambient conditions, and the bulk bandgap was estimated at 0.9 eV or 1.19 eV from two separate computational studies.10, 12-13 While doping, defect generation, size confinement and variation of atomic composition in a compound material can promote tuning of its bandgap within a limited range, the ability to fabricate Bi2Se3 with different crystal structures affords a wide range of properties and provide a unique opportunity to exploit this material in vastly different fields. Amongst the room temperature deposition techniques, chemical bath deposition (CBD), electrodeposition and successive ionic layer adsorption and reaction (SILAR) are the most common methods to fabricate films and nanoparticles on to a substrate. The experimental bandgaps reported for Bi2Se3 grown by CBD were found to be in the 1.4 – 1.7 eV range; however the crystal structure of the as-prepared material could not be precisely assigned.14 A lower bandgap has been observed for annealed samples (~130 – 200°C), which was associated to a more pronounced rhombohedral crystal structure.14-15 In the case of electrodeposited Bi2Se3, the crystallinity of as-prepared samples was noticeable, allowing the conclusion that a mixture of rhombohedral and orthorhombic phase, as well as an almost-pure orthorhombic phase, could be obtained; upon prolonged annealing the transformation to the rhombohedral phase was practically complete.10-11 Thus, room temperature synthesis of Bi2Se3 is believed to favor the growth of the metastable orthorhombic phase. In the case of SILAR deposited nanoparticles however, identification of the crystal structure from XRD patterns was not successful, and correlations among processing and materials characteristics are not yet fully understood.16 In this study, we investigate SILAR deposited Bi2Se3 to understand its compositional features, crystallinity, optical bandgap, band structure and effect of annealing of the material. The results obtained from this research further strengthens the hypothesis that a semiconducting Bi2Se3, suited to absorb visible light, can be prepared by room temperature deposition methods, opening opportunities in the field of solar energy conversion and photonic devices. Experimental Section Bi2Se3 deposition The deposition of Bi2Se3 was carried out using the SILAR method. The Bi-precursor solution was prepared by dissolving 0.05 – 0.15 M of Bi(NO3)3.5H2O (Alfa Aesar, 98%) in ethylene glycol (Spectrophotometric grade, 99%+) following literature; the pH of the solution was 0.1.1718 In order to achieve complete dissolution of the salts, the solution was continuously kept under stirring on a hot plate at 70°C for 30 minutes. The rinsing solution of ethylene glycol was very viscous and therefore the deposited substrates did not dry under nitrogen gas flow nor under heating (~60 – 70°C) by placing the substrate on a hot plate. A second ethanol (abs) solution was then used to rinse off the excess ethylene glycol after the first rinsing. Thus, after rinsing in ethylene glycol and ethanol for around 10 s and 20 s, respectively, the deposited substrates were completely dried under nitrogen gas flow before immersing into the Se-precursor solution. Se adsorption was performed by dissolving SeO2 and NaBH4 in pure ethanol (Fisher Scientific, 2


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anhydrous); this solution (pH = 10.4) was prepared in a three neck flask under nitrogen gas flow.19 Using a 1:2 ratio of SeO2 and NaBH4 ensured the formation of Se2- in the final solution under inert atmosphere as shown by Eq. 1.20 For example, to prepare a 0.15 M Se2- precursor solution, 0.15 M SeO2 was added to ethanol and left under stirring for 30 min or more until it completely dissolved. Later, 0.3 M NaBH4 was added to the solution, resulting in the color instantly turning to deep orange/red. After continuous stirring for 5 – 10 minutes under argon or nitrogen gas flow, the solution becomes transparent indicating that it is ready for use. The resulting Bi2Se3 deposit was rinsed in ethanol (abs) for 15 – 20 s and dried completely under nitrogen gas flow before the next immersion. SeO + 2NaBH + 6C H OH → Se + 2Na + 2BC H O + 5H + 2H O − − − 1

Synthesis of quantum dots (QDs) or nanoparticles by SILAR is usually carried out using precursor concentrations in the range of 0.02 – 0.05 M. In contrast, and in order to obtain higher surface coverages, more concentrated precursors of around 0.1 – 0.15 M have been used.21-23 For lower concentrations (0.05 – 0.1 M), 2 minutes of dipping time in each Bi- and Se-precursor solutions were used, while for the higher precursor concentration of 0.15 M, the substrates were dipped in the solutions for 1 min each. A sample deposited with a certain number of cycles, N is labelled as Bi2Se3[xN] throughout the rest of this manuscript. Bi2Se3 was deposited on the fluorine-doped tin oxide (FTO) side of a FTO coated glass (Sigma Aldrich, TEC7), while the glass side was first masked using a single sided Kapton tape (Ted Pella Inc.). After the depositions, the tape was peeled off from the glass side, ensuring that the absorbance values were accurately determined based on Bi2Se3 coverage of the FTO surface only. Material Characterization The morphology of the Bi2Se3 depositions was characterized by using FEI Helios G4 and FEI Quanta 650 Scanning Electron Microscopes (SEMs), while the elemental analysis was carried out by an EDS instrument attached to the SEM. The Bi2Se3 composition was determined using AZtec software where the areas under Bi and Se spectral features in the EDS spectrum were taken into account to calculate their respective atomic fractions from the total area under the Bi and Se spectra. Raman spectroscopy was performed on selected samples using a 514 nm LASER as the excitation source coupled to a Renishaw Raman spectrometer. XRD patterns were collected using a PANalytical X’Pert Pro MPD X-ray diffractometer using Bragg-Brentano (θ2θ) configuration with Cu Kα source of wavelength of 0.154 nm. The grazing incidence XRD (GI-XRD) measurements were performed on a PANalytical Empyrean X-ray diffractometer. The optical transmission and reflectance spectra for the bare FTO and Bi2Se3 deposited samples were measured using a Cary Varian 5E UV-VIS-IR Spectrophotometer. X-ray photoelectron spectroscopy (XPS) data were obtained using a monochromated X-ray source at a pass energy of 50 eV in an UHV system described previously.24 Spectral analysis was carried out using KolXPD software in which the spectral features were deconvoluted with Voigt line shapes.25 To describe the band alignment of the samples, ultraviolet photoemission spectroscopy (UPS) was performed in the same UHV system, using He I excitation and analyzer pass energy of 2 eV. Mott-Schottky analysis for Bi2Se3 was performed using a Bi2Se3[x45]/FTO as the working electrode in a three electrode setup at a frequency of 316 Hz. A phosphate buffer solution (0.08 3



M K2HPO4, 0.02 M KH2PO4 in water) having a pH of 7.4 was used as the electrolyte for the Mott-Schottky measurement.

3. Results and Discussions 3.1 Elemental Analysis and Morphology The SILAR depositions were carried out using 0.15 M precursor solutions to achieve a higher coverage of the substrate (FTO) surface. The deposits were in all cases found to be slightly Serich, specifically the Bi:Se atomic ratio was around 37:63 which was close to the ideal 40:60 composition of Bi2Se3. Fig. 1a – e shows the top-down morphology of a series of deposits on an FTO substrate due to the increasing SILAR depositions from 15 to 60 cycles. In the case of 15 cycles of deposition, the surface area of the FTO substrate was completely covered by Bi2Se3 nanoparticles. Between 30 and 45 cycles of deposition, the Bi2Se3 deposits joined together and form a thicker nanocluster network. After 60 cycles of SILAR depositions, the nanocluster network was observed to be more spread out on the FTO substrate. The increase in the amount of deposited Bi2Se3 could be confirmed from EDS elemental analysis and the relevant data are included in the supporting material.

Figure 1. SEM images (200 nm scale bar) of (a) bare FTO and gradual coverage of the FTO substrates by Bi2Se3 deposits using (b) 15 cycles (c) 30 cycles (d) 45 cycles and (e) 60 cycles. XPS analysis on the deposited surface of an as-prepared Bi2Se3 (45 deposition cycles) on FTO was carried out. The survey spectrum is shown in Fig. 2a, while the high resolution spectra with the fitted peaks for Bi 4f, Se 3d, O 1s and Na 1s are shown in Fig. 2b – e. The fitting of the Bi 4f spectrum revealed three chemical states with two different Bi oxidation states of Bi3+ and Bi5+, whose binding energies agree with a previous report.26 One of the Bi3+ oxidation state corresponded to Bi2Se3 bonding with Se (labelled Bi-Se). The second Bi3+ oxidation state was due to Bi-O bonding in Bi2O3. The percentage fraction of Bi2Se3, Bi2O3 and possibly Bi2O5 (arising from the Bi5+ oxidation state) were around 53%, 39% and 8%, respectively. The surface oxide formation occurred possibly during post-synthesis exposure to air. Aside from the Bi2O3, fitting of O 1s and Na 1s also revealed Na2O oxide formation on the surface. Since no Na was detected in the EDS spectra we speculate that this Na is highly surface localized. 4


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Figure 2. (a) XPS survey spectrum of Bi2Se3[x45]/FTO and high resolution XPS spectra of (b) Bi 4f (c) Se 3d (d) O 1s (e) Na 1s with fitted spectrum and associated bonds.

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3.2 Crystal Structure X-Ray Diffraction (XRD) patterns using the Bragg-Brentano configuration were collected for the as-deposited Bi2Se3 on FTO substrates but no distinguishable peaks could be observed for Bi2Se3 compared to that of a bare FTO, probably due to the limited equivalent thickness and the nanoscale dimensions of the deposited Bi2Se3 (Fig. S2 in SI). Alternatively, to account for the for low concentration of the deposited materials, grazing incidence XRD (GI-XRD) configuration was adopted to collect the XRD pattern of as-prepared Bi2Se3, shown in Fig. 3a. A total of four peaks could be identified in the GIXRD pattern out of which the peaks at 34.03° and 37.94° were due to the SnO2 (substrate). The two remaining peaks at 30.25° and 43.73° were attributed to the deposited Bi2Se3. The first Bi2Se3 peak at 30.25° closely matched the 30.68° peak of the orthorhombic Bi2Se3 phase. The reason for the observed left shift could be due to an overlap with a less intense 29.45° peak from the rhombohedral phase. The second Bi2Se3 peak centered at 43.73° was attributed to the 43.69° peak from the rhombohedral phase. The results from the GIXRD pattern conclusively shows the coexistence of rhombohedral and metastable orthorhombic phases in the SILAR deposited Bi2Se3.

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Figure 3. (a) XRD pattern for as-prepared Bi2Se3[x60]/FTO and (b) a typical Raman spectrum for SILAR deposited Bi2Se3[x40]/FTO and (c) a less frequently recorded different Raman spectrum for the same Bi2Se3[x40]/FTO. The Raman spectra for Bi2Se3 prepared using precursor concentration range of 0.05 – 0.15 M and deposition cycles between 25 and 90 were measured and are reported in Fig. 3b and 3c. No significant variations were observed among the spectra of the samples with different equivalent thicknesses, suggesting that the deposits show a similar crystallinity, both for smaller Bi2Se3 nanoparticles and the larger nanocluster network. Fig. 3b shows a typical Raman spectrum for a Bi2Se3 deposit after 40 cycles in 0.05 M precursor concentrations. Baseline correction was performed before fitting the data which revealed the existence of four peaks at 136.3 cm-1, 162.6 cm-1, 179.1 cm-1 and 250.8 cm-1. The first three were identified to be arising from E2g, A31 and A21g vibrational modes of Bi2Se3, respectively. The peak close to 250.8 cm-1 was assigned to the in-plane Se-Se vibrations.27 The E2g and A21g vibrational modes are the two commonly reported bulk phonon modes associated with the rhombohedral Bi2Se3.28-29 A second Raman spectrum recorded from the same sample under identical measurement conditions was rarely observed and is reported in Fig. 3c. The two, E2g and A21g, vibrational modes characteristic of the rhombohedral phase can be identified, while the A31 mode around 162 cm-1 was absent. The broad peak in Fig. 3b, covering the three Bi2Se3 vibrational modes (E2g, A31 and A21g), was due to the combination of an intense A31 mode combined with less intense E2g and A21g vibrations. In the case of single crystals and thin layers (< 7nm) of rhombohedral Bi2Se3, this A31 peak has been reported to be one of the six Raman active modes arising from surface vibrations.30-31 In the smaller nanometer dimensions, the surface to volume ratio of the nanostructures are higher than in bulk Bi2Se3 and such surface modes have been found to be more intense. Thus, the A31 mode could arise from surface vibration in nanostructures or from the metastable orthorhombic phase. We have not come across literature on theoretical studies that correlates the observed A31 vibrational mode specifically to the orthorhombic phase for Bi2Se3. The arrangement of Bi2Se3 atoms in a mixed phase structure most likely suppressed the vibrations from the E2g and A21g bulk phonon modes which are characteristic of the rhombohedral phase. In summary, the analysis of the Raman spectra was indicative of the presence of possible mixed phases of Bi2Se3 and a less frequent presence of sole rhombohedral crystals. 3.3 Optical bandgap and band alignment The transmission (%T) and reflectance (%R) spectra of Bi2Se3 deposited on FTO were measured to accurately determine the optical bandgap of Bi2Se3 deposits of different thicknesses. Each transmission spectrum was corrected for by the reflectance of that sample using Eq. 2 since with increasing number of SILAR cycles the reflectance of the deposited surface areas were observed to be different from that of a bare FTO.14, 21 The absorption coefficient, α was calculated using Eq. 3, where d is the thickness of the Bi2Se3 nanoclusters, %TBi2Se3/FTO and %TFTO are the corrected transmittance of Bi2Se3 obtained from Eq. 2 and transmittance of the bare FTO substrate, respectively.32 The bandgap, Eopt of Bi2Se3 was determined from the Tauc plots, (αhν)n vs. hν for 15 – 60 cycles.33 Nechaev et. al. have reported that Bi2Se3 has a direct bandgap and therefore an n = 2 value was used to calculate the Tauc plots shown in Fig. 4a.6 Table 1 shows 7



the bandgaps and estimated thicknesses for the particle clusters deposited in the 15 – 90 cycle range. The absorption cutoff for the Bi2Se3 prepared with 90 cycles of deposition was found to be 1.05 eV, which was only slightly lower than the 1.1 eV bandgap observed for 60 deposition cycles. This bandgap value of 1.05 eV probably indicated a lower limit for Bi2Se3 bandgaps prepared by SILAR. Also, the extended tail in the Tauc plot for the 90 deposition cycles (Fig. 4b) was not observed in the previous Bi2Se3 Tauc plots for 15 – 60 cycles (Fig. 4a). Such ‘tail effects’ resulting in a lower-than-bulk bandgap have been observed for other materials prepared by SILAR. This apparent lowering of the bandgap for SILAR deposition has been explained by Rabinovich and Hodes where the ‘tail effect’ was reported to dominate over size confinement effects due to the increase in crystal size.21 %T

100T − − − 2 100 − R

%T%&'(/*+, 1 α − ln $ - − − − 3 d %T*+, a (/

4πε ε 4 − − − 4 m∗ e

Table 1. Optical bandgap and related deposition parameters for Bi2Se3 deposited for 15 – 90 cycles. Deposition cycles 15 30 45 60 90

Thickness, d (nm) 18.9 23.9 29.5 48 92

Bandgap, Eopt (eV) 1.58 1.35 1.18 1.1 1.05

Figure 4. Tauc plots and corresponding bandgaps of (a) Bi2Se3 for 15 – 60 cycles of deposition and (b) for 90 deposition cycles showing the extended tail. 8


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The experimental bandgap of Bi2Se3 prepared by room temperature deposition techniques such as chemical bath deposition or electrodeposition have been reported to be around 1.7 – 1.4 eV and 1.25 – 0.8 eV, respectively.10, 14-15, 34 The bandgap values in the range of 1.58 – 1.1 eV as determined in our case match these previous reports. This range is higher than the predicted bulk Bi2Se3 bandgaps for both orthorhombic (~0.9 eV or 1.19 eV) and rhombohedral (~0.3 eV) crystal structures. The most likely factors that could contribute to the elevated bandgaps include the quantum confinement effect in nanoscale semiconductors, and possible variations in crystallinity. With regard to quantum confinement, the exciton Bohr radius for Bi2Se3 was calculated to be 52.5 nm using Eq. 4 (details in SI), where m*, e, ħ and εr and εo are the reduced effective mass of an exciton calculated from electron and hole effective masses, electron charge, reduced Planck’s constant and relative permittivity (=113 for Bi2Se3) and permittivity of free space, respectively.35 The estimated thickness of the Bi2Se3 deposits for 15 – 60 cycles (~18 – 48 nm) were lower than the calculated exciton Bohr radius and therefore an intermediate to strong size confinement effect could be expected in this case. However, the estimated thickness for 90 cycle deposition (~92 nm) was much higher than the exciton Bohr radius, and in this case we observed reaching a lower limit of bandgap at 1.05 eV for the Bi2Se3. This would be characteristic of bulk material properties where quantum confinement effect is diminished. Thus, from our study the 1.05 eV could be attributed to the bulk bandgap for the SILAR deposited Bi2Se3. Li et. al. have reported that the Fermi level (EF) of rhombohedral Bi2Se3 could be positioned inside the conduction band (CB), resulting in a degenerate semiconductor.36 The valence band maximum (VBM) for Bi2Se3 (45 cycles) were determined from both UPS and XPS measurements (comparison included in SI). The energy separation between the valence band maximum (VBM) and Fermi position (EF) determined from UPS data were found to be elevated due to the presence of a surface Bi2O3 layer. However, the VBM position of Bi2Se3 determined from XPS measurements was not affected by the presence of surface oxides (comparison included in SI) and we report the Fermi level here using XPS. Fig. 5a shows the VBM at 0.6 eV below the EF position (0 eV) determined from XPS suggesting a weak n-type conductivity. We determined a work function (φ) value of 4.19 eV (Fig. S5 in SI) which was much lower than the reported bulk Bi2Se3 work function of 5.0 - 5.4 eV.37-38 Upon exposure to air for very short times, a bilayer Bi termination (~0.7 nm) with metallic properties (Bi2-) has been reported to form on the surface which, combined with depletion of surface selenium, lowers the work function to that of metallic Bi (4.2 eV).37 No metallic Bi bonds could be detected in our XPS spectrum since it was beyond the resolution of our instrument. As the observed work function of 4.19 eV matched the metallic Bi (111) work function of 4.2 eV, we constructed a band diagram for Bi2Se3[x45], shown in Fig. 5b, depicting actual and observed Bi2Se3 work functions upon exposure to air. The flat-band potential (Vfb) and carrier density of Bi2Se3[x45] were determined from MottSchottky analysis, shown in Fig. 5c. The positive slope of the plot indicates an n-type conductivity of Bi2Se3, and the dopant density, Nd was around 9.2 x 1018 cm-3. This value is in contrast with the picture provided by XPS data, suggesting the presence of deep, localized donor levels from the oxide, resulting in electrically inactive defects. The Se antisite (SeBi) defects have been reported to dominate in Se-rich orthorhombic Bi2Se3, thus contributing mainly to the n-type 9



conductivity.38 On the other hand, Bi interstitial defects have been found to be the main contributing factor of high donor level in Se-rich rhombohedral Bi2Se3.39 The results from band alignment and Mott-Schottky analysis suggest that the SILAR deposited mixed phase Bi2Se3 behaves as an n-type semiconductor and does not conform to a semi-metallic topological insulator.

Figure 5. (a) VBM position measured by XPS, (b) schematic band alignment for the Bi2Se3[x45] and (c) Mott-Schottky plot for Bi2Se3[x45]. 3.4 Effect of Low Temperature Annealing Post-synthesis annealing at around 130 – 300°C of Bi2Se3 prepared by room temperature deposition methods have been reported to result in a phase transformation from a mainly orthorhombic structure to the rhombohedral structure.11, 14 Since ambient annealing would further oxidize the selenides, the Bi2Se3[x90]/FTO samples was annealed in vacuum at 100°C for 1 hour. The XRD pattern was collected using a grazing incidence configuration for the annealed sample, as shown in Fig. 6a. The five prominent peaks at 25.17°, 29.48°, 40.31°, 43.87° and 48.12° matched the reference XRD pattern for rhombohedral Bi2Se3. The two peaks at 33.95° 10


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and 37.99° were attributed to the SnO2 (substrate). None of the prominent peaks in Fig. 6a were attributed to the orthorhombic structure. In our previous study of annealing mixed phase Bi2Se3 films prepared by electrodeposition, we found that a complete transition to the rhombohedral phase occurs after 3 hours of annealing in 300°C.11 With a shorter duration (1 hr) of the low temperature (100°C) annealing in this study, the conditions were favourable to trigger the early stages of a similar phase transformation. Higher temperatures (~200 – 300°C) would further help the recrystallization towards a pure rhombohedral crystal structure. Raman spectra were also recorded to compare the effect of annealing. Fig. 6b shows the Raman spectra of both the as-prepared and annealed sample (Bi2Se3[x90]/FTO). The Raman spectra due to annealing resulted in two sharp and well-defined peaks from the bulk E2g and A12g vibrational modes at 132 cm-1 and 175.6 cm-1, otherwise dormant in the Raman spectra of the as-prepared Bi2Se3. These modes are characteristic of the rhombohedral crystal structure and indicate the improvement of the rhombohedral phase fraction and crystallinity of Bi2Se3. The decrease of the A31 mode around 162 cm-1 could be due to a change in the nano morphology of Bi2Se3 or could be intrinsic to the rearranged crystal structure. There was no significant change in the nanocluster morphology due to annealing from which we could correlate the decrease of the A31 mode to changes in size. The atomic rearrangement due to annealing and subsequent rhombohedral phase improvement most likely accommodated for E2g and A12g vibrations and a reduction of the A31 mode.

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Figure 6. (a) XRD pattern, (b) Raman spectra, and high resolution XPS spectra of (c) Bi 4f, (d) Se 3d for the annealed Bi2Se3[x90]/FTO sample. The high resolution XPS spectrum of Bi 4f for the annealed sample is shown in Fig. 6c, where the bismuth selenide to bismuth oxide ratio (54% to 46%) was comparable to the ratio of asprepared samples. However, there was no trace of a Bi5+ oxidation. Also, evidence of Se-O bonding was revealed from the Se 3d spectrum (Fig. 6d) which suggested that a possible oxide, such as Bi2Se3-xOx, could have formed on the sample surface. Bi2Se3 deposited after 90 cycles was studied to understand the effect of absorption cutoff due to annealing. After annealing the absorption cutoff for Bi2Se3 was found to be red shifted to 0.97 eV which can be correlated to the onset of growth of the rhombohedral phase fraction due to annealing. A more interesting effect was an increase in the tail region absorption showing a subbandgap absorption with a cutoff at 0.78 eV which is between the bulk orthorhombic (Eopt = 0.9 – 1.19 eV)) and bulk rhombohedral (Eopt = 0.3 eV) bandgaps. Depending on the relative phase fraction, these mixed phase Bi2Se3 bandgap could lie between these two extremes of bandgaps, and a gradual lowering of peak absorption with higher red shift can be expected. XPS measurements were carried out on Bi2Se3[x45]/FTO to observe any possible shift of the Fermi level position due to annealing, which could suggest changes in Bi2Se3 conductivity. Annealing of Bi2Se3 prepared by CBD has also been previously reported to increase dark current by several orders of magnitude.14 The fitted XPS spectrum for the annealed sample is shown in Fig. 7b where the VBM level was determined to be 0.65 eV below the Fermi position at 0 eV binding energy. The upward shift in Fermi level position is proportional to an increase in the density of conduction electrons in Bi2Se3. Due to annealing, the energy separation between EF and VBM was found to increase slightly which is consistent with our conclusion that the low temperature annealing triggered the early stage transition onset from semiconducting towards semi-metallic properties.

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Figure 7. (a) Variation in optical bandgap of Bi2Se3[x90] due to annealing and (b) VBM determined from the XPS spectrum of annealed Bi2Se3[x90]. 4. Conclusions A wet chemistry-based low cost and room temperature technique, SILAR, was used to synthesize Bi2Se3 nanostructures. By increasing the number of SILAR deposition cycles isolated Bi2Se3 nanoparticle deposits joined together and formed a nanocluster network achieving higher surface coverage. Determination of the crystal structure of the SILAR deposited Bi2Se3 was important to assess whether the materials are semiconducting (bulk Eopt ≥ 0.9 eV) or semimetallic (bulk Eopt ≈ 0.3 eV). Analysis of the GI-XRD pattern of the as-prepared Bi2Se3 was conclusive to determine a mixed phase crystal structure. Analysis of Raman spectroscopy revealed an intense A31 mode besides the less intense E2g and A21g bulk phonon modes that are characteristic of the rhombohedral phase. Further computational studies on the allowed bulk vibrational modes for a pure orthorhombic Bi2Se3 need to be carried out to correlate phase identification of deposits solely from the Raman spectrum. The optical bandgap values obtained for 15 – 90 cycles of SILAR depositions were found to be in the range of 1.58 – 1.05 eV. These variations were attributed to the quantum confinement effect observed in semiconductors where the energy needed to free the valence band electrons is higher than the bulk bandgap value. While computational studies are more common, schematic band alignment of Bi2Se3 from experimental studies are rarely reported, especially for room temperature deposition methods. We have taken a report by Gao et. al. on band alignment of semi-metallic rhombohedral Bi2Se3 as a reference for comparison.35 Using XPS measurements and bandgap data we have constructed a schematic band diagram of SILAR deposited Bi2Se3 which showed that the material exhibits n-type conductivity, further confirmed from MottSchottky analysis. Such findings have clearly demonstrated the possibility of fabricating a semiconducting Bi2Se3 via SILAR. These results allow us to generalize and further strengthen the hypothesis that room temperature deposition methods can be effective to synthesize semiconducting Bi2Se3 with bandgaps higher than 0.9 eV, possibly suitable for solar cell devices.

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Synthesis and Material Properties of Bi2Se3 Nanostructures

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