Quasi Two-Dimensional Luminescent Silicon Nanosheets - The

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Quasi Two-Dimensional Luminescent Silicon Nanosheets Debosmita Karar, Nil Ratan Bandyopadhyay, Ashit Kumar Pramanick, Debanjan Acharyya, Gavin J. Conibeer, Niladri Banerjee, Olga Kusmartseva, and Mallar Ray J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03988 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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The Journal of Physical Chemistry

Quasi Two-Dimensional Luminescent Silicon Nanosheets Debosmita Karar,1‡ Nil Ratan Bandyopadhyay,1 Ashit Kumar Pramanick,2 Debanjan Acharyya,3 Gavin Conibeer,4 Niladri Banerjee,5 Olga E. Kusmartseva,5 Mallar Ray1,*‡ 1

Dr. M. N. Dastur School of Materials Science and Engineering, Indian Institute of

Engineering Science and Technology, Shibpur, PO: Botanic Garden, Howrah – 711103, West Bengal, India 2

Materials Science and Technology Division, CSIR-National Metallurgical Laboratory, Jamshedpur - 831007, Jharkhand, India

3

Department of Electronics and Telecommunications, Indian Institute of Engineering Science and Technology, Shibpur, PO: Botanic Garden, Howrah – 711103, West Bengal, India

4

ARC Photovoltaics Centre of Excellence, University of New South Wales, Sydney – 2052, Australia 5

Department of Physics, Loughborough University, Loughborough LE11 3TU, United Kingdom

ABSTRACT Although predicted to be stable under ambient conditions, the experimental synthesis of silicene – the two-dimensional silicon analogue of graphene – has been a great challenge. Here, we report on the preparation of scalable quantities of crystalline nanosheets of twodimensional silicon by simple topo-chemical exfoliation of layered Zintl phases. The simple 1 ACS Paragon Plus Environment

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process leads to the formation of stacked layers of 2D Si nanosheets which are arbitrarily surface terminated with oxygen, hydrogen, hydroxide and other ligands. The nanosheets exhibit strong room-temperature photoluminescence and their overall spectroscopic characteristics closely resemble other forms of silicon nanocrystals. Remarkably, the pelletized nanosheets exhibit significantly high Hall mobility even in the absence of any doping or surface treatment, which is better than the reported values of doped Si nanosystems. Such ensembles of two-dimensional nanosheets of silicon bear signature of exotic electronic properties and represent important building blocks in two-dimensional electronics along with potential applications in solar cells, next generation thermoelectric materials and sensors.

* Corresponding author. Tel.: +91 33 2668 8140 E-mail address: [email protected] INTRODUCTION Silicene, the Si based counterpart of graphene, is a synthetic monolayer consisting of a twodimensional (2D) honeycomb arrangement of Si atoms. Silicene is expected to be stable under ambient conditions,1,2 and has been predicted to possess exotic properties such as Dirac band-structure,3 quantum spin Hall effect,4 chiral superconductivity,5 giant magnetoresistance,6 photo-induced topological phase transition,7 and many others. In fact, the slightly buckled ‘armchair’ form of silicene, where neighbouring Si atoms are displaced out of plane, is expected to bestow silicene with structural flexibility and electronic properties that are not found in planar graphene.2 Additionally, of course, silicene offers the advantage of easy integration with the existing Si based microelectronic technology.

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Despite all these theoretically predicted exotic properties, actual experimental development of free-standing silicene has remained a formidable challenge. It is well-known that Si atoms, unlike carbonaceous materials, tend to prefer sp3 over sp2 hybridization due to strong electron-electron correlation.8 Consequently, mechanical or chemical exfoliation of sp2 bonded material is not an option for synthesis of silicene.9 As SiO2 is a hard insulator, reduction of Si-oxide to obtain silicene is also not possible. Epitaxial growth of a 2D network of Si atoms on metal substrates has been the preferred route for development of silicene-like structures. Ag(111) is the substrate of choice for concomitant growth of differently buckled, sp2 and sp3 mixed arrangements of 2D Si atoms.10-16 Besides Ag(111), development of silicene has also been reported on Ir(111),17 ZrB2,2 and MoS2,18 substrates. However, the epitaxial growth of 2D Si on substrates is understandably a complicated process which requires ultra-high vacuum and expensive deposition techniques. All attempts to isolate the effect of the substrate from the overall properties of silicene have so far remained unsuccessful. Linear bands of silicene on Ag(111) have been observed experimentally, but it is not possible to unambiguously associate them with Dirac fermions. The measured Fermi velocity and the range of the linear bands are much larger than the theoretically expected values and the linear bands disappear when silicene is not on top of the Ag substrate. Hence, it is usually understood that the observed linear bands are either the sp bands of bulk Ag or are due to hybridization of silicene and Ag states.19-21 An inexpensive and easy route that avoids problems such as hybridization, is the chemical exfoliation of structures with layered Si, providing a much simpler strategy for obtaining quasi-2D sheets of Si. However, a layered 2D structure of Si is not easily found in naturally occurring Si based compounds. Among the different compounds of Si, silicides like CaSi2 consist of corrugated Si(111) planes sandwiched between Ca layers with interconnected Si6 3 ACS Paragon Plus Environment


rings. Preferential etching and isolation of Ca in CaSi2, serves as a plausible route to obtain scalable quantities of 2D crystalline Si. Of course, such simple processes tend to produce quasi 2D sheets of Si and are prone to random oxidation and surface attachments. Nevertheless, for nearly all practical purposes, quasi 2D Si sheets with arbitrary or selective surface functionalization are also very exciting. These quasi 2D structures serve as the backbone in exploring many fundamental properties and also for various applications, similar to their carbon based analogue – reduced graphene oxide. Following wet chemical methods, Nakano et al.22 exfoliated layered siloxene [Si6H3(OH)3] into individual nanosheets with a thickness of 0.7 nm and lengths in the range 100-200 nm. In the following year, the same group reported preparation of 2D Si layers by etching of Mg doped CaSi2 which restricted spontaneous oxidation.23 However, very little progress has been made in this direction ever since, possibly because of the ‘imperfections’ in the final structure. Stacking of the individual layers, oxidation and rupture of the sheets, make this material ‘non-ideal’ as compared to a fully sp2 bonded, 2D silicene. Nonetheless, these nonideal, quasi 2D structures are significant as they exhibit strong room-temperature PL and bear clear signature of remarkable Hall mobility, which we demonstrate in this work. We show that selective etching of layered CaSi2 can produce scalable quantities of such quasi 2D crystalline Si having promising optical and electronic properties due to the presence of a 2D network of Si atoms. EXPERIMENTAL Synthesis: About 1g of CaSi2 powder (Sigma Aldrich) was stirred continuously in 100 mL of 35% HCl with the aim of transforming the silicide to siloxene, following the prescription of Nakano et al.23 The temperature was maintained at 0-40C throughout the process. During this reaction, H2 gas evolved and black CaSi2 turned into a greenish yellow powder. The stirring 4 ACS Paragon Plus Environment

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was continued for nearly 72 hrs till evolution of H2 ceased completely, indicating complete transformation of CaSi2 to Si6H3(OH)3 following the reaction: 3CaSi2+6HCl + 3H2O → Si6H3(OH)3 + 3CaCl2 + 3H2

(1)

The mixture was filtered and the filtrate was washed with ethanol. A greenish yellow solid powder Si6H3(OH)3 was then precipitated from a translucent colloidal suspension. The reaction scheme is illustrated in Figure 1. For measurement of electronic properties, pellets of the synthesized powders were formed by pelletizing them in a hot press at 2200C under 5 tonnes/cm2 pressure. Following our earlier studies,24-25 it is highly unlikely that the nanofeatures of Si are altered after pelletization at such low temperature and pressure. After pelletization, aluminium contact pads were deposited on the tablets by thermal evaporation.

Figure 1: Schematic showing the chemical exfoliation of CaSi2 leading to the formation of stacked 2D silicon nanosheets with arbitrary surface functionalization. Characterization: XRD was performed using a Bruker D8 advanced powder diffraction instrument with CuKα1 radiation (1.54 Å) to investigate the crystallinity and phases of the powder samples obtained after wet chemical treatment of CaSi2. Field emission scanning electron microscopy (FESEM) was carried out with a Zeiss-Sigma microscope. The samples were imaged after drying on the copper grid in the bright field mode using a high-resolution transmission electron microscope (HRTEM), JEOL 2010. Atomic force microscopy (AFM) 5 ACS Paragon Plus Environment


imaging was carried out using an SPA 400, Seiko Japan in non-contact mode. To remove the maximum amount of volatile content in the sample, the samples were kept under the exposure of an incandescent lamp after depositing them on freshly cleaved mica surface. Background corrected Fourier transform infrared (FTIR) absorption spectra were recorded in the region 400–4000 cm−1 by a Shimadzu 84005 FTIR. X-ray photo-electron spectroscopy (XPS) measurements were conducted on an Omicron Multiprobe (Omicron NanoTechnology Gmbh, UK) spectrometer fitted with an EA125 (Omicron) hemispherical analyzer. Monochromatic Al-Kα source operated at 150 W was used and the pass energy of the analyzer was kept at 40 eV. A low-energy electron gun (SL1000, Omicron) with a large spot size was used to maintain charge neutralization. The voltage of the electron gun was fixed at −3 V. Raman spectra of the synthesized powder were obtained using an InVia (Renishaw) Raman spectrometer using a confocal microscope (50 Å objective lens) equipped with a single monochromator and a charge coupled device. A 514 nm wavelength Ar laser was used as the excitation source. The Raman mapping was performed in micro-Raman backscattering geometry using a LabRAM HR system of Horiba Jobin Yvon. The excitation light was provided by a He-Ne (632.8nm) laser. The laser light was focused onto the sample using a X50ULWD objective lens. The laser power on the sample surface was 3.8 mW. The backscattered Raman radiation was collected through the same lens and recorded using a 600 grooves/mm grating and a CCD detector (256 x 1024 pixels) with liquid-nitrogen cooling. The spectrometer was operated in the confocal mode with the entrance slit of 150 µm, the confocal hole of 400 µm and the focal length of 800 mm. UV-visible spectra were recorded by a JASCO V-750 spectrophotometer. A Horiba JobinYvon, Flurolog-3 (Nanolog) spectrofluorometer (model FL3-11) fitted with a 450 W monochromatized xenon source was used to acquire photoluminescence (PL) emission of the samples at room temperature. Hall


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mobilities and electrical conductivities were measured with an Ecopia HMS-3000, Hall effect measurement system.

RESULTS AND DISCUSSION Preferential etching of CaSi2 by HCl, followed by separation of the 2D crystalline Si backbone results in significant changes in the crystallographic phases as shown in the XRD profiles in Figure 2. The XRD pattern of as-received CaSi2 powder is characterized by multiple sharp peaks corresponding to reflections from the various crystallographic planes of crystalline CaSi2 — indexed and marked in Figure 2 (a) according to JCPD card number 752192. As expected, there is no signature of elemental Si in this case. Interestingly, all these sharp peaks disappear in the XRD profile of the synthesized powder [Figure 2 (b)], which now exhibits the presence of elemental crystalline Si. Diffraction features corresponding to reflections from (111), (220) and (311) planes of Si are clearly identifiable (JCPD card number 03-0529). An amorphous hump extending from, 2θ ≈ 150 to 300, is also present in the synthesized sample, indicating the presence of amorphous oxide alongside the crystalline component. Although it is not possible to assert about the planar nature of the sample from powder XRD data, the observed changes in the diffraction features is distinctly different from the amorphous-like XRD patterns reported earlier,22-23,26 and unambiguously establish the transformation of CaSi2 to crystalline Si. Broadening of the diffraction peaks is also apparent, suggesting crystal size reduction along with accumulation of microstrain in the synthesized sample. At this stage we refrain from any quantitative estimation of size from the broadening data since most of the widely used methods for estimation of average size from XRD data assume a nearly homogeneous size effect in all dimensions and hence cannot be used in this case.27 Here, we simply restrict ourselves to the claim that Si in crystalline form is present in 7 ACS Paragon Plus Environment


the synthesized sample and note that the XRD pattern of the sample is different from the diffraction profiles of exfoliated siloxene and layered polysilanes reported previously in the literature.23,26

Figure 2: XRD profiles of (a) the as-received CaSi2 starting material and (b) the synthesized powder of chemically exfoliated Si nanosheets derived from CaSi2. The plate-like features of exfoliated CaSi2 are distinctly visible in the electron micrographs shown in Figures 3 (a)-(d). Stacks of thin flake-like structures piled up adjacent to one another can be seen in the FESEM images shown in Figures 3 (a)-(b) and in Figure S1 (a) [Supplementary Information]. An approximate estimate of the thickness of the flakes is made from the magnified image [Figure 2 (b)], which is found to be ~20 nm, suggesting that the resolvable flakes in the micrographs are formed by piling of several layers of 2D Si nanosheets. The micrographs also reveal that the lateral dimensions of the plates extend up to several micrometers.


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Figure 3: Electron micrographs of stacks of 2D crystalline nanosheets. (a) FESEM image showing piled up flake-like structures; (b) higher magnification FESEM image showing nearly parallel stacking of nanosheets; (c) HRTEM micrograph of the exfoliated nanosheets revealing graded contrast variation due to stacking of sheets on top of one another. The corresponding SAED pattern is shown as an inset; (d) higher magnification TEM image showing resolved lattice fringes of the (111) exposed plane of Si nanosheets. Moiré fringes formed due to non-accordant overlap of the crystalline sheets are marked for clarity. The bright field HRTEM images shown in Figure 3 (c) and in Figures S1 (b)-(c) [Supplementary Information] further clarify the formation of layers of 2D sheets of Si. The piling of individual sheets to form quasi 2D flakes can be clearly identified from the graded contrast variation due to stacking of sheets on top of one another. The selected area electron 9 ACS Paragon Plus Environment


diffraction pattern (SAEDP) shown as inset Figure 3 (c), consists of diffuse diffraction spots forming ring-like patterns along with a halo which is typical of randomly oriented nanocrystalline samples. The diffraction rings corresponding to reflections from (111), (200) and (311) planes of Si are discernible and are marked for clarity. The high magnification HRTEM image shown in Figure 3 (d) reveals the crystalline fringes corresponding to the (111) exposed plane. In addition to the lattice fringes we can see relatively broader dark and bright bands which are the Moiré bands formed due to overlap of two or more crystal motifs. The observed Moiré bands are not parallel to the lattice fringes suggesting a non-accordant overlap of the nanosheets.28 The presence of Moiré pattern strongly suggest that the individual nanosheets pile up on one another forming a stack of 2D nanosheets of crystalline Si. To investigate the surface ligands which are expected to be bonded to these crystalline nanosheets, standard FTIR absorption studies were carried out and a typical spectrum is shown in Figure 4. The absorption bands corresponding to the surface ligands present in the sample are indicated in the spectrum. Interestingly, these surface ligands are typically present in nearly all forms of Si nanostructures, but are not present in CaSi2.29 The absorption band appearing at 3420 cm-1 is due to hydrogen bonded Si-OH bond stretching,30 and the peak appearing at 2108 cm-1 is due to Si-H stretching in SiH2, while the shoulder at 2254 cm-1 is due to Si-H stretching of O3SiH.31,32 Si-H bonds also result in the absorption peak observed at 812 cm-1 for the Si-H2 rotational mode.33 As expected, absorbed water contributes to the wellknown band of molecular bending vibrations of H-O-H at 1631 cm-1.34 Very weak features at 2928 and 2861 cm-1 may be attributed, respectively to asymmetric and symmetric stretching vibrations of CH2.35,36 The presence of very small but identifiable features corresponding to the bending vibrations of methylene and methyl groups are present at 1380 cm−1 and 1460 cm−1, respectively.37 The presence of methylene and methyl groups is due to remnant alcohol 10 ACS Paragon Plus Environment

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that was used to wash the sample. A dominant and sharp absorption feature peaking at 1052 cm-1 is assigned to the bond stretching of Si-O-Si,38 indicating a dominant presence of oxide on the surface of the nanosheets. However, the absence of the O2Si≡Si–H mode is significant as it implies that the interlayer Si–O–Si bonding is not present in the sample. The most important feature of the FTIR spectrum is the appearance of a peak at 517 cm-1, which is representative of Si-Si bonds confirming the existence of Si-Si network.39 The FTIR spectrum therefore reveals that besides the Si-Si network, several surface species are bonded to the 2D nanosheets. Presumably, the unsaturated bonds on Si surface produced during the synthesis process attaches to nearly all reactive species producing a surface terminated with multiple moieties.

Figure 4: FTIR spectrum of the sample showing the Si-Si bond along with several other bonded surface species. XPS study was performed to understand the distribution of the oxidation states of Si in the nanosheet (Figure 5). Four distinct peaks centred around 102, 155, 285 and 532 eV are clearly seen in the full scan, Figure 5 (a). These peaks correspond to Si 2p, Si 2s, C1s and O1s states, respectively. To obtain a clearer idea as to the oxidation states of Si, the 2p and 2s 11 ACS Paragon Plus Environment


peaks were deconvoluted as presented in Figure 5 (b) and (c), respectively. Four peaks are found from the deconvolution of Si core level 2p lines around the binding energies at 97.5, 99.7, 102.9 and 103.5 eV, respectively. We are unable to identify the oxidation state of Si responsible for the peak at 97.5 eV, but the three other peaks are related to specific binding energies for Si0, Si+2 and Si+3 oxidation states, as indicated in Figure 5 (b).40 Information regarding XPS of the Si/oxide interface for Si 2s is rare in literature. While the peak at 150.8 eV and 154 eV can be tentatively associated with the Si0 oxidation state, the peak at 149.1 eV cannot be identified with a specific oxidation state at this stage of investigation, although their existence matches well with literature.40 Nevertheless, the envelope of the deconvoluted peaks provides an excellent fit to the experimentally obtained curve.

Figure 5: XPS spectra of the exfoliated 2D nanosheets (a) full range scan, (b) and (c) deconvoluted spectrum of the Si 2s and 2p peaks, respectively. AFM studies of thoroughly sonicated samples deposited on cleaved mica were carried out in non-contact mode. Figure 6 (a) and (b) are representative topographic images of the sample where stacked layers of the sample can be identified on the mica background. The line profiles corresponding to the marked regions in Figure 6 (a) and (b) are shown in Figure 6 (c) and (d), respectively. From the micrograph shown in Figure 6 (a) and the corresponding line profile [Figure 6 (c)], we see a flat profile of the mica substrate that is followed by a region 12 ACS Paragon Plus Environment

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where the thickness is ~0.8 nm, which possibly corresponds to a surface functionalized single layer of 2D Si. Subsequently, the profile sharply peaks to ~12.4 nm in the region where the contrast increases in the topography image, suggesting that the region is made of several stacked 2D sheets. A high resolution scan of an area where the thickness of the film is ~0.8 nm is shown as inset of Figure 6 (a). Although we do not see a regular honeycomb structure extending throughout the micrograph (due to the random surface attachments) there are some regions where hexagonal lattice arrangements are discernable. In the high resolution AFM micrograph shown in Figure 6 (b) and in the corresponding line profile [Figure 6 (d)], we see a remarkably similar trend. Corrugations of different heights ranging from ~0.7 nm to ~3.1 nm relate well with layers of quasi 2D sheets of Si. Here, too we see signature of honeycomb arrangement, though not very distinct. Regions with sub-nanometre thickness possibly correspond to a single buckled layer of imperfect 2D silicene, although such assertions cannot be made conclusively at this stage of the investigation. Overall, the sample consists of stacked layers of multiple sheets which are formed due to collapsing of the exfoliated Si sheets on one another. The finding of AFM therefore agrees very well with the results of the TEM investigations.



Figure 6: AFM investigation of Si nanosheets: (a) topography showing two different layers of the sample; inset shows a high resolution scan where an area showing the honeycomb arrangement is marked; (b) high resolution AFM micrograph showing corrugated features corresponding to different layers of quasi 2D Si; (c) and (d) are the line profiles taken along the marked red lines in (a) and (b) respectively, indicating the formation of stacked multilayers. Raman spectroscopy is one of the most powerful tools for analyzing 2D materials and has been widely used to understand silicene like structures. Silicene has three acoustic phonon branches — flexural acoustic (ZA), transverse acoustic (TA) and longitudinal acoustic (LA) and three optical phonon branches — flexural optical (ZO), transverse optical (TO), and longitudinal optical (LO).41 The calculated non-resonant Raman spectrum of free standing, defect-free, silicene predicts a main G-like peak at around 575 cm-1 corresponding to the degenerate in-plane LO and TO phonon modes at Γ point.42 However, this expected intense 14 ACS Paragon Plus Environment

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peak at ~575 cm-1 has not yet been experimentally observed. Many groups have observed and theoretically predicted the Raman modes of silicene grown epitaxially on Ag(111), which are strongly influenced due to Si-Ag interaction.43-46 Both ex-situ and in-situ (no ambient oxidation) studies have been performed and a wide variety of Raman spectra have been reported. It is not meaningful to compare our spectrum with any of these reports since Raman spectra of epitaxial silicene is significantly different from free standing single layer or multilayer silicene due to strong Si-substrate inteaction.42,46-47 Moreover, as our sample surface is somewhat arbitrarily terminated with oxygen, hydrogen, hydroxide and other ligands, the agreement with the Raman modes of ideal free standing silicene is also not expected.

Figure 7: (a) Raman spectrum of the exfoliated Si nanosheets for three repeat scans; and (b) Raman mapping spectra for 36 different points showing a clear evolution of the D peak. The left inset is the optical micrograph of the sample with the sampling pixels shown in different colours, and the right inset shows the Raman spectra collected at 5 different points with the peak positions marked for clarity. The overall Raman spectrum of the sample and the Raman mapping of a small area of the sample are shown in Figures 7 (a) and (b), respectively. The Raman spectrum of the



exfoliated 2D nanosheets consists of a nearly symmetric peak at 510 cm-1 along with features at higher as well as at lower wavenumbers, whose peak positions are indicated in the figure for clarity. The main peak at 510 cm-1 is shifted towards lower wavenumbers compared to bulk c-Si for which the LO phonon mode corresponds to a Raman peak at 520 cm-1. Such a large shift of ~10 cm-1 cannot be explained by full sp3 nanocrystalline Si. Only small ∼2 nm (3D confined) nanocrystals are expected to induce the observed shift,47,48 whereas, our samples clearly have lateral dimensions extending up to several micrometres. For 2D Si nanostructures a significant lower wavenumber shift of the bulk c-Si Raman peak has been reported for defective and oxidized Si structures.39,42,49 Scalise et al.42 predicted that in hydrogenated or defective Si nanoribbons a dominant D peak appears in the range of 510-520 cm-1 corresponding to the higher frequency phonon branch at the K-point, while the intensity of the G-like peak falls drastically. We believe the observed peak at 510 cm-1 with an FWHM of ~25 cm-1, corresponds to this D peak. Similar to our findings, Fuchs et al.39 also noted a Raman mode at 515 cm-1, which they assigned to vibrations of interconnected Si planes of siloxene following calculations done by Kanellis et al.49 The ~25 cm-1 peak-width is also in excellent agreement with the predicted width of the D peak of non-spherical nanostructures.39 A dominant D peak appearing in the range of 500 to 520 cm-1 due to defective or hydrogenated 2D sheet is further corroborated by the Raman mapping data shown in Figure 7 (b). As shown in the optical micrograph [left inset, Figure 7 (b)], a small region was selected on the sample surface and Raman data were collected for 36 different positions. We clearly see that the peak position varies within the range of 518 and 499 cm-1 for different points on the sample. The intensity of the peaks as well as their widths increase as the peak shift towards lower wavenumbers. This strongly suggests that at points where piling of sheets occur, the Raman signal is obtained close to the well-known 520 cm-1 peak. However, when the Raman signal is collected from regions where the thickness approaches that of a single 16 ACS Paragon Plus Environment

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but defective monolayer, the peak shifts towards lower wavenumbers and widens. For at least one point, the black curve in the Figure 7 (b), there is no peak in this region whereas a distinct kink appears around 637 cm-1 due to oxides of Si. The other smaller features present in the Raman spectrum corresponds to different modes present in 2D Si and oxides of Si. The small kink at ~420 cm-1 has been reported to be present in all forms of Si nanosheets with defects and absent in perfect silicene, but its symmetry is not yet understood.50 The feature appearing at 292 cm-1 is close to the 295 cm-1 D3 peak of silica.51 Raman band centre at 600 cm−1 can be attributed to different structural units such as rings of 3- and 4- membered SiO4-tetrahedra,52 broken Si–O bonds,53 or overcoordinated Si and O.54 Similarly, the peak centred at ~795 cm-1 corresponds to LO-TO vibrations of SiO2.55 It may be noted here that most of the oxide related Raman features are very dominant in purely oxidized samples. The low intensity of these features suggests that oxide, though present, is not able to dictate the Raman features of our sample. A relatively more prominent feature around 930 cm-1 is reported to arise from processes involving 2 TO phonons in c-Si.55 The high energy peak at 1095 cm-1 is also due to TO phonons of surface Sioxide. However, this mode is expected to be obscured by the band owing to the interstitial oxygen in a SiO2 network.56 The absence of this obscuring band tentatively suggests that oxide present in the sample is restricted to the surface. The analysis of Raman spectra of our sample therefore re-affirms the formation of 2D hexagonal c-Si ring like structures along with surface modification, primarily with oxygen and hydrogen.



Figure 8: Co-plot of normalized UV-visible absorption and PL from the exfoliated nanosheets and as-received CaSi2. The PL of CaSi2 is indistinguishable from noise and is not shown. PL spectra were obtained by exciting the sample with 325 nm excitation. The overall structural features of the prepared samples establish the formation of quasi 2D networks of crystalline Si, which are somewhat arbitrarily passivated by a variety of surface ligands. Now, such a structure is expected to entail carrier confinement with serious implications on the spectroscopic features of the sample. It is well known that light emission from Si nanostructures have attracted immense attention and different forms of nano-Si have been widely investigated in this regard. However, despite more than two decades of intensive research on light emitting Si nanostructures, the issue of the light emission mechanism still awaits a consensus.57-60 Albeit the existing controversy, the phenomenon of light emission is very exciting as it opens up the possibility for various applications, particularly in optoelectronic and sensing. Keeping the above in mind, the UV-visible absorption and PL emission spectra of the asreceived and chemically exfoliated CaSi2 powder were studied and are shown in Figure 8. The absorption profile of CaSi2 is more-or-less featureless and similar to previous reports.61,62 18 ACS Paragon Plus Environment

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An optical bandgap of ~3.18 eV corresponding to the absorption edge at ~390 nm is in perfect agreement with the estimated direct bandgap of 3.1 eV reported by Sakanaka et al.62 The absorption spectrum of the exfoliated sample on the other hand exhibits some very interesting features. First, the profile is distinctly different from that of CaSi2. Second, the spectrum has features that closely resemble other forms of nanocrystalline Si.63-64 The shoulder appearing at ~280 nm has been frequently observed in absorption spectra of Si nanocrystals and has been tentatively attributed to confinement effects.65-66 Third, the rough estimates of the direct (~ 4.2 eV) and indirect (~2.9 eV) bandgaps made from Tauc’s analysis,67 and point of inflection,68 are far removed from the bandgaps of bulk Si or that of bulk siloxene, which has a bandgap of 2.4 eV.23 Fourth, a broad hump extends from 300 to 500 nm in the absorption characteristic suggesting availability of states in the visible region. All these remarkable spectral changes upon exfoliation are likely to be associated with size quantization and effects of additional surface/interface states. However, at this stage we refer only to the possibilities and refrain from making any assertive proposals regarding the electronic band-structure. Nevertheless, we can conclude that CaSi2 has been converted into a form having absorption features similar to nanocrystalline Si indicating some manifestation of size quantization. As mentioned earlier, we will not delve into the mechanism of light emission here but would like to point out that while the exfoliated nanosheets exhibit intense room temperature PL that can easily be seen with the unaided eye, the recorded PL of CaSi2 is indistinguishable from noise. The PL profile (Figure 8) of the sample in nearly Gaussian, peaking at 540 nm under an excitation of 325 nm. Whether this luminescence is due to inter-band recombination of quantum confined excitons or due to transitions from oxide related interface states or due to the presence of surface species, warrants a separate investigation.



Finally, to understand the collective charge transport through the ensemble of 2D nanosheets, Hall measurements were performed on the pelletized samples by an Ecopia HMS-3000, Hall effect measurement system. A 1 mm thick circular pellet having a diameter 13 mm was mounted on a four-probe spring board and Hall measurements were performed in a closed

chamber under magnetic flux density of 0.556 T using van der Pauw method. A photograph of the set-up along with the sample with contacts is provided in the supplementary information file (Figure S5). Needless to mention, any measurement done on the pellets will depend on the pellet features (porosity, particle size and distribution, interfaces, contacts, etc.) and reflect an average behaviour of the arbitrarily functionalized nanosheets. The value of resistivity bulk carrier concentrations, sheet carrier concentrations, and average Hall coefficient were found to be 7.24 Ω.cm, 2.341 x 1016 per cm3, 2.341 x 1011 per cm3 and 2.66 x 102 cm3/C, respectively. It is worth mentioning here that the negative sign of the bulk carrier concentrations, sheet carrier concentrations, and average Hall coefficient indicates an n-type conductivity of the 2-D Si nanosheets. The measured Hall mobility (µH) and electrical conductivity (σ) of the pelletized nanosheets are presented in Table 1 and compared with some reported mobility and conductivity values of other pelletized Si nanostructures. From Table 1, we first observe that the Hall mobility of our sample has a remarkably high value of 36 cm2/Vs, which is higher than nearly all other reports involving other forms of Si nanostructures. Secondly, we note that the conductivities of the pellet prepared with the nanosheets are similar to the values reported in literature for quasi zerodimensional Si nanocrystals. The substantially higher electrical conductivity (~103S/cm) reported by Yusufu et al.69 and Claudio et al.70 correspond to samples prepared by spark plasma sintering in an inert atmosphere, which ensured greater compaction and prevented oxidation. Additionally, both the samples were “super-heavily doped”.70 Heavy doping along with high temperature sintering is expected to produce a significant improvement of 20 ACS Paragon Plus Environment

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conductivity and mobility. Yet, we see the Hall mobility of our sample is remarkably high compared to the others, barring the super-heavily P doped sample. In the absence of any surface treatment and with pelletization performed at around T = 2200C and P = 5 tonnes/cm2, the pellets are bound to have local high potential barriers, both due to oxides and due to porosity. The heights and widths of these barriers are also expected to be significant for this kind of sample. Consequently, a Hall mobility better than other forms of pelletized Si nanostructures can only be qualitatively accounted by the presence of a Si-Si 2D skeleton. Expectedly, proper surface treatment, doping and improved pelletization will yield samples with much improved, if not exotic, values of σ and µH, which can have very important implications for next generation high efficiency thermoelectric materials and solar cells. These aspects warrant separate investigations. Table 1: Electrical conductivity (σ) and Hall mobility (µH) of pelletized Si nanostructures compared with the results of this work:

Sample Crystalline Si:H films (size:~3.7 nm)

σ(S/cm) ~10-2 to 10-1

(best results)

Pelletized Si NCs (size: ~44 nm)

140 to 505

Heavily P doped Si nano-precipitates

1.8×10 to 3 2.52 ×10

Pelletized Si nano-powder

10

Pelletized nanosheets (undoped)

1.38×10

µH (cm2 V-1s-1 )

Reference

5 to 10

J Appl. Phys. 75, 797, 1994.71

4 to 15

J. Appl. Phys. 110, 113515, 2011.72

43

Nanoscale, 6, 13921, 2014.69

15.1

PCCP 16, 25701, 2014.

36

Present work

3

3

-1

70

CONCLUSIONS



In summary, we have synthesized stacked layers of c-Si 2-D nanosheets by topochemical exfoliation of layered CaSi2 and carried out an exhaustive investigation of their structural, optical and some electronic properties. Microscopic and X-ray diffraction investigations reveal the formation of predominantly crystalline 2D layers, which tend to pile up on one another, along with some exposed monolayers. Analyses of the IR absorption, X-ray photoelectron emission and Raman modes confirm the formation of 2D hexagonal c-Si ring like structures along with somewhat arbitrary surface termination with oxygen, hydrogen, hydroxide and other ligands, which get attached spontaneously during the exfoliation process. The synthesized nanosheets exhibit strong room temperature luminescence in the blue-green region and their overall spectroscopic character is similar to other forms of Si nanocrystals. An investigation of the overall electrical conductivity and Hall mobility of the pelletized samples revealed that their electronic properties are also comparable with other Si nanosystems even in the absence of any surface treatment or doping. The Hall-mobility of the crudely prepared pellets is found to be better than other reported Si nanostructure based pellets, which opens the scope for application of these nanostructures for developing high efficiency thermoelectric materials and efficient solar cell materials. ASSOCIATED CONTENT Supporting Information containing: i) Additional electron micrographs of quasi 2D Si nanosheets, ii) Additional Raman spectroscopy results, iii) XPS spectrum of O1s, iv) Calculation of PL quantum yield (QY), v) Details of Hall measurements AUTHOR INFORMATION Corresponding Author *Email (M. Ray): [email protected] 22 ACS Paragon Plus Environment

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ORCID ID: Mallar Ray: 0000-0001-8173-1857 Author Contributions ‡

D. Karar and M. Ray have contributed equally. All authors have given approval to the final

version of the manuscript. Funding Sources IIEST, Shibpur regular research grant Notes The authors declare no competing financial interest. ACKNOWLEDGMENT DK acknowledges the support of IIEST, Shibpur PhD fellowship. The work of DA was supported by CSIR, Government of India, under a Ph.D. Fellowship We all thank Professors Tarun Mondal and Sugata Ray and Mr. Indranath Bhowmik of IACS, Kolkata for extending their support to record XPS data of our samples. We thank Yicong Hu and Rob Patterson of UNSW for assistance with Raman measurements. We acknowledge the Centre of excellence on microstructurally designed advanced materials, IIEST, Shibpur for allowing us to use the Hall measurement set up. NB acknowledges funding from the UKIERI grant managed by the British Council. REFERENCES: 1.

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Figure 1: Schematic showing the chemical exfoliation of CaSi2 leading to the formation of stacked 2D silicon nanosheets with arbitrary surface functionalization. 379x152mm (96 x 96 DPI)

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Figure 2: XRD profiles of (a) the as-received CaSi2 starting material and (b) the synthesized powder of chemically exfoliated Si nanosheets derived from CaSi2. 270x223mm (300 x 300 DPI)


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Figure 3: Electron micrographs of stacks of 2D crystalline nanosheets. (a) FESEM image showing piled up flake-like structures; (b) higher magnification FESEM image showing nearly parallel stacking of nanosheets; (c) HRTEM micrograph of the exfoliated nanosheets revealing graded contrast variation due to stacking of sheets on top of one another. The corresponding SAED pattern is shown as an inset; (d) higher magnification TEM image showing resolved lattice fringes of the (111) exposed plane of Si nanosheets. Moiré fringes formed due to non-accordant overlap of the crystalline sheets are marked for clarity. 130x129mm (120 x 120 DPI)



Figure 4: FTIR spectrum of the sample showing the Si-Si bond along with several other bonded surface species. 279x215mm (300 x 300 DPI)


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Figure 5: XPS spectra of the exfoliated 2D nanosheets (a) full range scan, (b) and (c) deconvoluted spectrum of the Si 2s and 2p peaks, respectively. 382x128mm (120 x 120 DPI)



Figure 6: AFM investigation of Si nanosheets: (a) topography showing two different layers of the sample; inset shows a high resolution scan where an area showing the honeycomb arrangement is marked; (b) high resolution AFM micrograph showing corrugated features corresponding to different layers of quasi 2D Si; (c) and (d) are the line profiles taken along the marked red lines in (a) and (b) respectively, indicating the formation of stacked multilayers. 227x198mm (96 x 96 DPI)


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Figure 7: (a) Raman spectrum of the exfoliated Si nanosheets for three repeat scans; and (b) Raman mapping spectra for 36 different points showing a clear evolution of the D peak. The left inset is the optical micrograph of the sample with the sampling pixels shown in different colours, and the right inset shows the Raman spectra collected at 5 different points with the peak positions marked for clarity. 304x126mm (96 x 96 DPI)



Figure 8: Co-plot of normalized UV-visible absorption and PL from the exfoliated nanosheets and as-received CaSi2. The PL of CaSi2 is indistinguishable from noise and is not shown. PL spectra were obtained by exciting the sample with 325 nm excitation. 296x209mm (300 x 300 DPI)


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Quasi Two-Dimensional Luminescent Silicon Nanosheets - The

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