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Shining Light on New Generation Two Dimensional Materials in Computational Viewpoint Arkamita Bandyopadhyay, Dibyajyoti Ghosh, and Swapan K. Pati J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00044 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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The Journal of Physical Chemistry Letters

Shining Light on New Generation Two Dimensional Materials in Computational Viewpoint Arkamita Bandyopadhyay,†,# Dibyajyoti Ghosh‡,# and Swapan K Pati†,§,* †

New Chemistry Unit, ‡Chemistry and Physics of Materials Unit and §Theoretical Sciences Unit,

Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064, India #

A.B and D.G. contributed equally

*Corresponding author: [email protected]

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ABSTRACTS Energy and sensing related applications using two-dimensional (2D) materials with tuneable optoelectronic properties has been a hot topic of research. The genres of 2D materials grow every day leading to new possibilities into optoelectronic devices. In this perspective, we have discussed in a nutshell several impacts of light-matter interactions in new generation 2D materials. Using reliable computational approaches, in-depth understanding about the fundamental optical absorption and emission character as well as further prediction of the potential applications for these materials in the field of photovoltaics and sensing have been explored. Various modifications of the parent 2D-materials by computational-designing with enhanced performance have been investigated to guide the experimental efforts. The major computational challenges and their probable solutions for the 2D-material based optoelectronic research have also been briefly outlined. TOC GRAPHICS

KEY WORDS 2D materials, Optical properties, Photocatalytic activity, Solar cell, Sensor, Computational study


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DISCUSSIONS Two-dimensional materials with their extraordinary structural, electronic, mechanical and optical properties have revolutionized the research field of nanotechnology.1-3 Tuneable band gap, high carrier mobility, easy incorporation of spin-centres in these materials emerges them as the promising candidates for nanoelectronics and spintronics.1 Beside these, a huge research effort has also been directed in search for superior optoelectronic and photovoltaic properties of the parental as well as modified 2D-materials. Large surface area, reduced photogenerated electron-hole recombination, high visible light absorbance make these nanomaterials ideal for the next-generation energy-conversion devices.1,

3

Despite all these

suitable properties, the first generation parental 2D-materials such as pure graphene (zero band gap)1 and hexagonal boron nitride (hBN, wide band gap insulator)3 exhibit poor performance in these energy-applications due to lack of semiconducting nature. As a result, different chemical and physical modifications of these materials along with exploration of new generation 2D-materials are in the lime-light of current nanomaterial research.4 Tuning the size, shape, impurities and functional groups of these new generation 2D-materials can lead to the tailoring of band gap and absorption coefficient in them.5 The efficient absorption of photons of a wide frequency-range from microwave to ultraviolet from the solar spectrum results in their applications in the fields of solar cells, light emitting diodes, optical detection, photocatalysis etc.5-7 In this Perspective, we briefly discuss the potential photocatalysis, energy-conversion and sensing related applications of recently emerged 2D-materials from a computational point of view along with few relevant experimental evidences. Note that, as the electronic and optical properties of graphene, h-BN and transition metal dichalcogenides have been reviewed by several groups recently,5-8 we keep these materials out of our discussion.



Figure 1. Top and side view of different newly explored 2D sheets: (a) g-C3N4, (b) phosphorene, (c) silicene, (d) germanene, (e) stanene and (d) Ti2CO2 MXene sheets. Black, blue, orange, yellow, purple, green, cyan and red balls represent C, N, P, Si, Ge, Sn, Ti and O atoms, respectively. Computational Approaches. Reliable computational prediction of optical properties for these 2D-materials is quite challenging due to several factors such as their complex selection-rule, dominant excitonic nature.5 Broadly, two theoretical methodologies have been developed over the years, (1) many-body perturbation theory based8-9 and (2) time-dependent density functional theory (TDDFT) based approaches. The perturbation based method is three-step process where initially using different levels of density functional theory (LDA, GGA, Hybrid etc), the Kohn-Sham (KS) orbitals are calculated which provides singleparticle Green’s function, prerequisite for the GW-approximation.8-9 Next, exchanging the KS exchange-correlation potential with electron self-energy, the quasi-particle band structure energies are obtained under GW-approximation.9 Finally, the four-point Bethe-Salpeter equation (BSE) is solved to treat the electron-hole interaction.9 This procedure is a welldeveloped approach for yielding improved macroscopic dielectric function and consequently evaluating the absorption spectra which exhibit excellent agreement with the experimental data. However, solving the BSE involves diagonalization of a large matrix, substantially increasing the computational demand and consequently makes the approach suitable only for small systems.9 Another widely used method, TDDFT, is formulated as the time-dependent 4 ACS Paragon Plus Environment

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counterpart of the KS theorems.5 Under the linear perturbation theory and with the help of Casida formalism, one can obtain the optical excitations in molecules and solids.10 Apart from these methods, real time propagation of KS orbitals or the RT-TDDFT method has also been proved to be very useful in gaining microscopic insight of the changes in the electronic density of states in real time scale.11 More detail of these approaches can be found elsewhere.5, 8-11 For the calculation of emission properties in 2D materials DFT and the GW-Bethe Salpeter equation (BSE) method has been used to simulate the quasiparticle bandstructures, exciton energies etc.12 Fermi’s Golden rule has been used to derive the radiative decay rate of an exciton in this method.12 Optical absorption and emission. Before exploring different applications, we briefly discuss the optical absorption and emission properties of these 2D-materials in their parent as well as in modified form. These 2D-materials in general are excellent absorber of light due to large density of states at band-edges or high oscillator strength for interband transitions.5 However, the frequency at which the materials will absorb the light entirely depends on the electronic band structure of the systems.5

For example, graphitic carbon nitride (g-C3N4) sheets,

because of the presence of relatively high band gap (2.7eV), absorbs in ultra-violet (UV) region of the solar spectrum (see Figure 1a). Computationally, the optical properties of thses 2D systems are highly dependent on the exchange-correlation functionals used. The calculated optical gap with

Perdew–Burke–Ernzerhof (PBE) form of GGA functionals

suggests a much lower value of 1.19 eV for g-C3N4 compared to the experimental absorption studies.13 As LDA+U methods do not work in these weakly correlated covalent systems, rigid scissors operation correction13 or using an hybrid functional (e.g. Heyd-Scuseria-Ernzerhof or HSE) is necessary to reproduce accurate optical gap. The lowest optical transitions in g-C3N4



is from occupied N pz orbitals to empty C pz* orbitals.14-15 Due to its inefficient absorption of solar light in pure sheets, many computational physicochemical modifications has been proposed on g-C3N4 sheets, such as changes in shape, size, functionalization and doping to tune its adsorption characteristics (see Figure 1b). All these methods were proved to be potential ways to modify the optical absorption profile of g-C3N4 sheets covering UV to infrared (IR) range.8 Transition metal (TM) doping in g-C3N4 sheets has also been found to be very efficient to enhance the low energy optical absorption in C3N4 sheets due to a pπ of C/N to empty d-orbitals of TM transition.16 Interestingly, other newly emerged 2D material such as phosphorene17 (i.e. in this case single layer black phosphorus) exhibits an much smaller optical gap of 1.22 eV in G0W0+BSE level of theory, bit lower than experimental one (1.88eV).18 Most importantly, it exhibits strong optical anisotropic nature along armchair and zigzag directions (see Figure 2(c)), which remain intact after chemically doping the system.19 Wu et al. have shown boron-doped phosphorene to absorb more intensely in lower energy range (low light absorption in these systems is observed between 0 and 3.0 eV in the zigzag direction which is absent in the pure phosphorene ) than pure phosphorene due the presence of a borondominated impurity band or charge transfer (CT) state in this modified system.19 Fortunately, recent experimental developments are now able to realize and testify the computational calculations. For example, time-resolved diffuse reflectance spectroscopic measurement (TDR spectroscopy) has proved to be useful to analyse the kinetic mechanisms and confirmed the efficiency of covalently bonded P in g-C3N4 network or black phosphoreneC3N4 heterostructures as efficient photocatalyst for hydrogen evolution reaction (HER) in visible to near infrared (NIR) region by Zhu et al.20 Recently, luminescence behaviour of these sheets has also been well explored.21-22 Beside these exceptional absorption properties, these new generation 2D-systems also exhibit interesting emission properties. Tuneable emission spectra due to size-dependent quantum confinement effect of g-C3N4 nanosheets


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makes them suitable for UV-blue and white LED emitter applications.23 The large band gap along with the potential of interlayer coupling leads to the tuneability in their emission properties.23 Similarly, phosphorene quantum dots were also found to show intense and stable photoluminescence in the blue-violet region.24

Figure 2. Optical absorption spectra of different 2D sheets. (a) Experimentally obtained UVvisible spectra of g-C3N4 sheets in DMSO medium (reprinted with permission from ref.12) (b) optical absorption spectra of g-C3N4 nanoflakes after physicochemical modifications calculated by TDDFT with HSE06 (reprinted with permission from ref 8), showing a huge range of absorption wavelength, (c) optical absorption spectra of pure and B doped phosphorene along armchair and zigzag directions in PBE-GGA level (reprinted with permission from ref

19

), (d) optical conductivity of germanene, antimonene and their

heterostructures PBE-GGA level (reprinted with permission from ref

25

), (e) imaginary part

of dielectric functions for 0% and 2% strained stanene in various levels of theory, (reprinted with permission from ref

26

) and (f) plot of the imaginary part of the dielectric

constant of Ti2CO2 and Zr2CO2 MXenes in the PBE-GGA level of theory (reprinted with permission from ref 27). Other mono-elemental 2D materials, such as silicene (monolayer of silicone atoms)28 also exhibit interesting features in its optical transitions. The absorption spectra of silicene



contain two major peaks, a low energy peak at 1.74 eV and a broad high energy peak at 4-10 eV. These two peaks arise due to a transition from π to π∗ states (which also compares fairly well with the HSE06 calculated band gap of the system), and lower lying σ states to π∗ states, respectively.29 Another novel 2D material, germanene (monolayer of germanium) exhibits the presence of Dirac cone at high symmetry K-point like graphene.30 Furthermore, its optical property also can be tuned in presence of substrate or doping.30 Germanene-antimonene (monolayer of antimony) heterostructures exhibit an enhanced optical response, due to the presence of CT states, in the visible light region of the solar spectrum, resulting in a better candidate for photo-devices.25 Analogous to other ultrathin, low band gap 2D materials, stanene (monolayer of tin), at a GW+BSE+SOC (spin orbit coupling) level, shows an absorption edge at approximately 0 eV and the intense first peak arises at 0.31 eV,26 suggesting efficient IR absorption property of the material. It is noteworthy that, in absence of the SOC effect, though the electronic nature of stanene modifies a bit, the optical transition character remains almost unaltered.26 Computational calculations on 2D SnSe and GeSe sheets in GW level suggest strong visible light absorption in these materials as well.31 Another class of new generation layered material, MXene (where M is an early transition metal and X is C/N atoms) is also not lagging behind in the race. Electronic properties of MXenes range from metallic to semiconductor depending upon M, X or surface terminating group, T in the system,32 as a result, can exhibit a huge range of optical absorption profile.27 Different TM containing MXenes, e.g. Ti2CO2, Zr2CO2, Hf2CO2, Sc2CF2, Sc2C(OH)2, and Sc2CO2 are found to be semiconducting in nature and demonstrate interesting optical properties.27 Zhang et al. have demonstrated that particularly Ti2CO2, Zr2CO2 and Hf2CO2 2D sheets show efficient visible light absorption even in the PBE level of theory.27 Also, number of “n” layer in a Mn+1XnT2 may tune the optical nature of these systems. Further, suitable functionalization such as the fluorination and hydroxylation of Ti3C2 systems exhibit


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smaller absorption and reflectivity compared to pristine or oxygenated systems, making them better candidates as transparent electrodes.33 Recently, Wang et al. have experimentally synthesized a few representative ultrasmall MXene systems, including Ti3C2 sheets, and demonstrated strong optical absorption and photoluminescence behaviour in these systems.34 Photocatalytic applications. One of the most exciting applications of new generation 2D-materials is their probable usage as the efficient photocatalyst. In the recent years, the search for clean and renewable energy sources, especially efficient transformation of solar to chemical energy has become a subject of intense research.6, 8 Apart from different other ways (e.g. solar or wind energy conversions etc.), hydrogen evolution reaction (HER) provides us one such clean energy source, H2 from water, widely named as water-splitting reaction. This reaction, performed in presence of a semiconducting photocatalyst under ambient condition and solar light, proves to be a pragmatic approach towards fighting future energy crisis.7 Few important criteria for a semiconductor to be used as an ideal catalytic surface for this reaction are stability under ambient condition, appropriate band gap and band alignment, small exciton binding energy, low electronic-hole recombination rate and efficient absorption of solar light.7 Easily tuneable optical gap in 2D systems along with their high surface area and good chemical stability in water medium, makes them class of candidate material worth to explore further. Considering HER of H2O as reaction of interest, the photocatalyst needs to be with optical band gap of more than or equal to 1.23 eV as well as should have proper band-alignment. The reduction and oxidation levels for H2O should be located inside the optical gap of the photocatalyst to perform both halves of the reaction (see Figure 3) on the catalytic surface. As µeH+ /H2 is equal to -4.44 eV and µhO2/H2O = −5.67 eV, the band-edges of the 2D photocatalyst for HER should be aligned accordingly (see Figure 3).8, 35 Thorough computational screening based investigations have been directed to find most suitable 2D-material with appropriate band-structure.7 As the exact band gap values and 9 ACS Paragon Plus Environment


alignment of valence and conduction band are most basic criteria here, beyond LDA/GGA based DFT computations such as GW approximation and hybrid functionals like HSE, have been adopted frequently.7 One of the most widely explored 2D-materials for photocatalytic water-splitting is monolayer of g-C3N4 sheet, it possesses an experimental as well as computational (HSE06 functionals based) band gap of 2.7 eV, along with favourable band alignment for HER (See Figure 3). These properties make the sheet an ideal material for water splitting reactions.2, 8, 36 Further to make g-C3N4 sheet more efficient visible light photocatalyst with fine-tuned optical absorption spectra, various physicochemical modifications have been proposed recently.8 Especially substituting sulphur in place of carbon or nitrogen in this 2D-network tune the optical band gap of the system to make the reaction more facile by providing singly occupied energy band near H+/H2 potential.8 Recent experimental study by Jourshabani et al. on mesoporous sulphur doped g-C3N4 network has also proved them to be excellent visible light absorber and photocatalytic remover of methyl orange.37-38 Nitrogen vacancy in these sheets has also been able to modify the band structure by introducing new defect induced energy levels in these systems, making them effective towards visible light photocatalysis of water splitting or CO2 reduction reactions.39 Combined computational and experimental study shows composites of g-C3N4 nanosheets and MoS2 or nitrogented graphene oxides, obtained by covalent crosslinking, can show upto ~246 times increased activity towards photocatalytic water splitting reactions compared to pristine g-C3N4 sheets due to the presence of CT states.40 Attaching functional groups, such as phosphates, on g-C3N4 surface also can find its application in photocatalytic CO2 reduction reactions due to similar band alignment modifications.41


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Figure 3. Schematic diagram of the two halves of HER and valence band and conduction band positions of different 2D sheets (discussed in this perspective) used as HER photocatalysts. The reduction potential of H+/H2 and the oxidation potential of O2/H2O have been denoted by blue and red dotted lines, respectively. Considering another promising material, phosphorene, as can be seen in Figure 3, only one half of the water splitting is possible on its surface.42 To overcome this limitation, various computational-design have been reported, namely, armchair or zigzag edge functionalization by CN or OCN groups in phosphorene nanoribbons (PNR) to achieve full water splitting reaction (by edge electric dipole layer induced modification of band edge positions),42 strain induced band gap and band edge alignment in the nano sheets etc.43 Note that, though detailed experimental proofs of phosphorene catalysed water splitting reactions are yet to be confirmed,44 black phosphorous-TiO2 hybrids have been already found to be effective photocatalyst in environmental and biomedical fields.42, 45 Most recently, HSE06-functionals based computational study has suggested different MXenes, particularly 2D form of Zr2CO2 and Hf2CO2 qualifies as excellent photocatalysts for water splitting or pollutant removal reactions due to their ideal band gaps and band edge positions.27, 46 A series of experiments have also exhibited the effectiveness of CdS/Ti3C2 co-



catalyst system towards enhanced exceptional HER mechanism. The rapid conduction of photoinduced electrons from the CdS to Ti3C2 due to the metallic nature of carbide is found to be the atomistic origin here.47 The fast-conducting photoinduced electrons on the O terminated Ti3C2 surface can immediately reduce the protons in the aqueous solution to evolve H2 gas.47 Silicene and germanene, due to their smaller band gap, are not very effective photocatalysts, however, CdS-MXene type electron transfer mechanism works for TiO2 and hydrogenated or functionalized silicene or germanene hybrid materials.48 Hence these functionalized 2D-systems can also be used as effective photocatalysts as predicted by the recent hybrid HSE functionals based computations.48 Similarly, other 2D-materials, like, monolayer and bilayer ZnSnN2, single layer t-ZnS, germanium monochalcogenide, phosphorous nitride, InSe family monolayers etc. have also been studied computationally and found to possess appropriate band edge position and band gap for water splitting, opening up new possibilities in the world of 2D photocatalysts.49-52 Solar cell applications. Beside photocatalytic applications, tuning of optical absorption of 2D materials has also finds its application in the field of solar cell research. Efficient IR wavelength absorption by the active absorber layer is a necessary condition for the efficient solar cells (see Figure 4 d).8, 53 In this regard, modification of the shape, size and functionalization of g-C3N4 sheet to absorb the wide range of solar-spectra has been explored thoroughly by both computational and experimental approaches.8, 53 As parent phosphorene is not stable under ambient condition, computational studies have predicted different ways to stabilize this 2D-sheet and have further explored its various usages in the solar cells. For example, performing hybrid HSE06 functionals based calculations, Hu et al. has predicted that differently edge-modified phospherene nanoflakes can act as effective donor and acceptor materials for a heterojunction solar cell by suitably modifying their HOMO-LUMO gap.54 In a heterojunction solar cell, the band alignment between the donor and acceptor 12 ACS Paragon Plus Environment

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material can be of three types, type I, type II and type III, among which type II junctions are proved to be very useful in high efficiency solar cells.55 In this type of junction, there can be two operative processes, (1) one photon mediated single electron transfer to the conduction band minima (CBM) from the intermediate band and one photon mediated hole transfer from the intermediate band to the valence band maxima (VBM) (total two photon process) or (2) one electron-hole pair creation by one photon (one photon process).55 A bilayer phosphoreneMoS2 system, due to their appropriate band gap (type II, see Figure 4) is found to be suitable for solar cell application.56 Recent experimental photovoltaic device fabrication with ultrathin nanosheet of black phosphorus indeed confirms these computational predictions.57 Furthermore, using global optimization algorithm combined with first-principles calculations, Guo et al. have explored the possibility of formation of many different phases of stable multilayer silicene and demonstrate that few of them can be used in the solar cells.58 Also, very recently, HSE06+SOC based calculation exhibits that antimonene can absorb in the IR range of the solar spectrum due to their low band gap, predicting it to be a potential candidate for the absorber material for the solar cells.59 All these newly emerged 2Dmaterials broaden up the world of the nanomaterial based solar cell research.



Figure 4. (a) Computed band edge positions of monolayer MoS2 as acceptor and AA-stacked bilayer phosphorene as donor. (b) VBM and CBM positions of monolayer MoS2 as acceptor layer and AB-stacked bilayer phosphorene as donor layer. (c) Plot of the imaginary part of the dielectric function computed using HSE06 functional and (d) power conversion efficiency contour in phosphorene-MoS2 solarcells as a function of the donor bandgap and conduction band offset (reprinted with permission

56

), (e) Temperature dependent photoluminescence

(PL) and (f) variation of integrated intensity of PL with temperature of g-C3N4, showing the temperature sensing ability of the sheet (reprinted with permission 60). Optical Sensors. Optical property of a 2D-material is very sensitive to their physical or chemical environments and this can lead to designing of nanomaterial based optical sensors.61-63 High surface area and consequent possibility to interact with a large number of molecular species makes these systems efficient towards optical sensing. g-C3N4 again proves to be an excellent candidate for different sensing purposes. Temperature dependent photoluminescence spectra has been used to show a wide range low temperature sensing property of g-C3N4 sheets (see Figure 4(e-f)).60 Also, this nanosheet has been used to fabricate arrays showing enhanced electrochemiluminescence and good sensitivity towards cholesterol sensing at single cell level.64 Further, phosphorene has been found to be excellent optical sensor for toxic gases like PH3 and AsH3, which has been designed based on inclusion of vacancy defect in phosphorene.65 Recent experimental study, combined with firstprinciples calculations and statistical thermodynamics modelling, suggests that the gas sensing ability of phsphorene is layer dependent and works best for a 2D system of 4.8 nm.66 Other than these few selected examples, abounding computational, experimental or combining studies suggest the efficient optical sensing of harmful, toxic gases or biomolecules by germanene,67 MXene68 and stanene69.

Summary and Future outlook: Due to their extraordinary optical property and possible applications in the energy and sensing related fields, the 2D-materials are in the lime-light of nanomaterial research. In this Perspective, we focus on the computational explorations 14 ACS Paragon Plus Environment

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of absorption and emission properties of very recently emerged next-generation 2Dmaterials. Potential applications of these parent as well as suitably modified materials in the field of photovoltaics and sensing have been discussed with an in-depth atomistic understanding about the origin of their exceptional performances. Computational investigations of fine tuning the shape and size, nature and extent of functionalization, concentration of defect-states are demonstrated as the potential approach to enhance the efficiency of these materials for different applications. Evidently, the reliable computational guidance to synthesize 2D-materials with outstanding optical property and promising performances has been proven to be major factor for the unprecedented advancement of this field. Despite of the remarkable advancement, still several challenges remain in the field of optoelectronics of 2D-materisl from a computational point of view. To discover the efficient candidates for various optoelectronic applications by exploring huge elemental space, machine-learning based screening method needs to be developed. As the accurate modelling of optical absorption by well-developed theories is still computationally demanding, we need to search for alternate cheap but reliable approaches. As the performance of 2D-materials is highly sensitive to the detail of environmental conditions, the inclusion of various factors such as solvents, temperature, gas-molecules and devicesurface with the expense of moderate cost is another growing field of computational material modelling. With the recent miraculous progress in both computational power and theoretical methods, we will certainly be able to cross these hurdles in near future.



Acknowledgements: SKP acknowledges DST, Govt. of India for funding Biography: Dr. Arkamita Badyopadhyay has recently completed her Ph.D from New Chemistry Unit, JNCASR. Her research interest spans around computational investigations of optoelectronic properties of low dimensional materials.

Dr. Dibyajyoti Ghosh was a PhD student at JNACSR, Bangalore. He received his doctorate degree in 2016 under the supervision of Swapan K Pati. He is currently working as a research associate at University of Bath, UK. His research focuses on first-principle computation of energy related materials. (Present Address: Dept. of Physics and Dept. of Chemistry, University of Bath, UK)

Prof. Swapan K. Pati is currently a Professor in the Theoretical Sciences Unit, JNCASR, Bangalore. His research interests compasses theoretical understanding of microscopic structure-property relationship for a wide spectrum of applications in transport, optical, magnetic, electrical and mechanical behaviour of low dimensional materials. He is also interested in state-of-the-art numerical methodology development.

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