Strain-tunable electronic properties and quantum capacitance of ScHfCO₂ MXene as supercapacitor electrodes

Abstract

MXenes have wide applications in energy storage devices because of their compositional diversity. Electronic and optical properties, Bader charge and quantum capacitance of Janus ScHfCO₂ MXene under biaxial strain are studied by density functional theory (DFT). The substitution of Hf atoms induces the decrease of the band gap of ScHfCO₂, which changes from direct semiconductor into indirect semiconductor. Band gap generally increases with the increase of the tensile strain because of the blueshift of Sc-d and Hf-d orbits, and ScHfCO₂ changes to M→K indirect semiconductor at $+$5% strain. ScHfCO₂ under strains from −5% to $+$4% maintains the indirect bandgap characteristics. The appearance of built-in electric field in ScHfCO₂ under strain improves the charge redistribution across Janus layer. ScHfCO₂ under compressive strain has better conductivity than ScHfCO₂ under tensile strain. ScHfCO₂ under strains are all promising cathode materials. Larger voltage improves the character of cathode materials because of their much larger |Q_c| when compared with those at aqueous system.

Graphical abstract

Keywords

Janus MXene / electronic properties / quantum capacitance / density functional theory / charge transfer

1 Introduction

Supercapacitor is a new type of energy storage devices and has the advantages of high power density, good stability, and low pollution [1, 2]. These excellent properties make it widely utilized in microgrids, portable devices, energy storage systems and electric vehicles [3−7]. However, low energy density becomes the main factor limiting the development of supercapacitors [8, 9]. Electrode material is an important part of supercapacitors and can affect its performance. Increasing quantum capacitance (C_diff) of electrode materials can improve the energy density of supercapacitor.

Graphene is a potential electrode material of supercapacitor with high theoretical specific surface area and high C_diff [10−15]. The application of graphene in semiconductors and supercapacitors is limited by its zero band gap [16]. Graphene-like, Xenes, MXenes and other two-dimensional (2D) materials are potential energy storage materials.

MXenes is a 2D transition metal (TM) carbide or nitride, which is obtained through etching A layer from MAX phase, M for the TM, X for C or N, and A for the IIIA or IVA group elements. The surface of MXenes is often bonded by O, OH, and F functional groups during etching. So MXenes have the formula of M_n+1X_nT_x, where n = 1, 2, 3, T_x represents the surface functional group [17−20]. Functional MXenes have potential applications in photocatalysis [21−23], sensors [24−26], energy storage devices [27−29]. The first synthesized MXene, Ti₃C₂T_x, is proved to have better capacitive properties than carbon materials [30]. The design and manufacture of supercapacitor electrodes based on MXenes has attracted much attention and has been rapidly developed [31].

Janus can describe novel structures of materials having two faces at the nanoscale [32, 33]. Janus MoSSe was successfully obtained in 2017 [34, 35]. Janus 2D materials have been shown to have unique properties both experimentally and theoretically [36, 37], and their potential applications have attracted the great interest of researchers [38]. Since then, the family of Janus 2D materials has expanded, such as Janus graphene [39], Janus TM chalcogenides [40, 41], and Janus MXenes [42, 43].

Janus-structured Co-Ti₃C₂ MXene is synthesized and demonstrated that its carrier separation efficiency was improved with the introduction of Janus structure [44]. Gao et al. [45] studied the structure of Janus M'MCO₂ (M' and M stands for V, Cr, and Mn) MXenes and reported their band gaps and magnetic moment. Janus MXenes have higher surface redox activity and significantly improved the charge storage capacities [46]. Janus ZrTiCO₂, ZrHfCO₂, and HfTiCO₂ have high hole mobility, photoinduced hole oxidization capability, and spatially separated electron-hole pairs, which indicates their possible application in high-efficiency photocatalytic processes [47]. Applying biaxial strain can regulate the structural and electronic properties of materials [48, 49]. Sc₂CO₂ was transformed from semiconductor to metal under ‒2% strain [50], and is a potential SO₂ gas sensor because of its high selectivity and sensitivity [51]. Hf₂CO₂ is a classical MXene with low thermal expansion coefficient and thermal conductivity [52], which make it have promising application in optoelectronics [53].

Inspired by the superior performance of 2D Janus materials, and Sc₂CO₂ and Hf₂CO₂ MXenes, we investigated the electronic and optical properties, C_diff of Janus ScHfCO₂ MXenes under biaxial strain by density functional theory (DFT) in this work. Work function (WF) and Bader charge are also investigated. The work can provide theoretical guidance for expanding the potential application of MXenes in optoelectronic devices.

2 Calculation details

All calculations are based on DFT [54] and are carried out with VASP code under PAW method [55]. 2 × 2 × 1 Sc₂CO₂ supercell is constructed, consisting 8 Sc atoms, 8 O atoms, and 4 C atoms. ScHfCO₂ is formed by replacing four Sc atoms in Sc₂CO₂ MXene with Hf atoms. The GGA-PBE functional is used [56]. Energy cutoff is 520 eV. The vacuum layer is set to 30 Å to avoid the interaction between adjacent slabs. The convergences of total energy and force on each atom are 10⁻⁶ eV and 0.01 eV/Å, respectively. 8 × 8 × 1 and 31 × 31 × 1 k-points are utilized for geometric optimization and electronic structure calculation, respectively. DFT-D3 method was used to modify the Van der Waals (vdW) interaction [57].

For the low-dimensional materials as electrodes, C_diff can be obtained by the following equation:

(1)$C_{\mathrm{d}\mathrm{i}\mathrm{f}\mathrm{f}}={\displaystyle \frac{\mathrm{d}\sigma }{\mathrm{d}{\varphi }_{\mathrm{G}}}}=e^{2}DOS(-Ve),$

where $\mathrm{d}\sigma$ and $\mathrm{d}{\varphi }_{\mathrm{G}}$ are the differentials of local charge density and local potential, respectively. The electrochemical potential shifts with $e{\varphi }_{\mathrm{G}}$, and the excess charge density can be deduced by [58]

(2)$\mathrm{\Delta }Q={\displaystyle {\int }_{-\mathrm{\infty }}^{+\mathrm{\infty }}D(E)\left[f(E)-f(E-e{\varphi }_{\mathrm{G}})\right]\mathrm{d}E,}$

where D(E) represents the DOS of the studied system, f(E) is the Fermi−Dirac distribution function, E is the relative energy with reference to the Fermi level, and e is the elementary electric charge. For 2D materials, C_diff can be obtained by the following equation [59]:

(3)$C_{\mathrm{d}\mathrm{i}\mathrm{f}\mathrm{f}}=e^2{\displaystyle {\int }_{-\mathrm{\infty }}^{+\mathrm{\infty }}D(E)F_{\mathrm{T}}(E-e{\varphi }_{\mathrm{G}})\mathrm{d}E,}$

where F_T(E) is the thermal broadening function and is obtained by [60]

(4)$F_{\mathrm{T}}(E)=(4{\kappa }_{\mathrm{B}}T{)}^{-1}{\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{h}}^2\left({\displaystyle \frac{E}{2{\kappa }_{\mathrm{B}}T}}\right).$

Here, ${\kappa }_{\mathrm{B}}$ is the Boltzmann’s constant, and T is 300 K. The storage charge on the MXene-based electrode’s surface is obtained by

(5)$Q={\displaystyle {\int }_0^{{\varphi }_{\mathrm{G}}}{C}_{\mathrm{d}\mathrm{i}\mathrm{f}\mathrm{f}}\mathrm{d}\varphi .}$

Cohesive energy (E_coh) is used to predict the relative stability of materials and is calculated by

(6)$E_{\text{coh}}={\displaystyle \frac{E_{\mathrm{t}\mathrm{o}\mathrm{l}}\left({\mathrm{S}\mathrm{c}\mathrm{H}\mathrm{f}\mathrm{C}\mathrm{O}}_2\right)-n_{\mathrm{S}\mathrm{c}}{E}_{\mathrm{a}\mathrm{t}\mathrm{m}}(\mathrm{S}\mathrm{c})-n_{\mathrm{H}\mathrm{f}}{E}_{\mathrm{a}\mathrm{t}\mathrm{m}}(\mathrm{H}\mathrm{f})-n_{\mathrm{C}}{E}_{\mathrm{a}\mathrm{t}\mathrm{m}}(\mathrm{C})-n_{\mathrm{O}}{E}_{\mathrm{a}\mathrm{t}\mathrm{m}}(\mathrm{O})}{n_{\mathrm{S}\mathrm{c}}+n_{\mathrm{H}\mathrm{f}}+n_{\mathrm{C}}+n_{\mathrm{O}}}},$

where E_coh is the total energy of Janus ScHfCO₂, n_Sc, n_Hf, n_C, and n_O are the numbers of Sc, Hf, C, and O atoms in unit cells, respectively, and E_atm is the energy of the corresponding free atoms. Negative E_coh indicates the structural stability of material.

Biaxial strain is applied as

(7)$\epsilon =\left(a-a_0\right)/a_0\times 100\mathrm{\%},$

where a and a₀ are the lattice constants of ScHfCO₂ under and without biaxial strain. In this work, the applied biaxial strain ranges from ‒5% to 5%.

3 Results and discussion

3.1 Structural property

Pervious research [61] indicates that the stable configuration of Sc₂CO₂ is that two terminated O layers locate asymmetrically on Sc and C atom on each side. Based on the configuration, we consider four possible configurations for Janus ScHfCO₂, which are shown in Fig.1. The result indicates that model III has the lowest energy, indicating the most stable structure of model III. Therefore, we will focus on the investigation on model III in the following work.

Fig.1 Four possible configurations of ScHfCO2: model I (a), model II (b), model III (c) and model IV (d). Purple, green, brown, and red represent scandium, hafnium, carbon, and oxygen atoms, respectively.

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Tab.1 lists the lattice constants and the related bond lengths of Janus ScHfCO₂. The corresponding parameters of Sc₂CO₂ and Hf₂CO₂ are also included in order to have a comparison. Fig.2(a) shows the corresponding atomic labels. The calculated lattice constants of Sc₂CO₂ and Hf₂CO₂ are close to the previous results [62]. The substitution of Hf atom decreases the lattice constant of Sc₂CO₂. Substitution atoms may result in charge redistribution, which affects the interaction between atoms and produce the change in bond length. Compared with Hf₂CO₂, Hf‒C and Hf‒O bond lengths decrease, possibly because Sc atom has stronger metallicity than Hf atom, and accepts more charge, which induces the decrease of Hf‒C and Hf‒O bonds. Hf substitution also results in the increase of Sc‒C bond length and the decrease of Sc‒O bond length.

Tab.1 Calculated lattice constants (Å), related bond lengths (Å) for Sc2CO2, Hf2CO2, and ScHfCO2.

System	a (Å)	$d_{{\mathrm{S}\mathrm{c}}_4-{\mathrm{C}}_4}$ (Å)	$d_{{\mathrm{H}\mathrm{f}}_4-{\mathrm{C}}_4}$ (Å)	$d_{{\mathrm{S}\mathrm{c}}_4-{\mathrm{O}}_8}$ (Å)	$d_{{\mathrm{H}\mathrm{f}}_4-{\mathrm{O}}_8}$ (Å)
Sc2CO2	6.850	2.200	−	2.094	−
Hf2CO2	6.442	−	2.363	−	2.133
ScHfCO2	6.653	2.250	2.293	2.075	2.106

Fig.2 Atomic label (a), bond lengths (b) and binding energy (c) of ScHfCO2 under strain.

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Fig.2(b) plots the related bond lengths of ScHfCO₂ under strain. We note that $d_{{\mathrm{S}\mathrm{c}}_4-{\mathrm{O}}_8}$, $d_{{\mathrm{H}\mathrm{f}}_4-{\mathrm{O}}_8}$, $d_{{\mathrm{S}\mathrm{c}}_4-{\mathrm{C}}_4}$, $d_{{\mathrm{H}\mathrm{f}}_4-{\mathrm{C}}_4}$ under strains from ‒5% to +5% gradually increase, while C‒O bond length gradually reduces. Fig.2(c) plots the E_coh of Janus ScHfCO₂ under strain. Negative E_coh of ScHfCO₂ under strains from ‒5% to +5% indicates its structural stability. Molecular dynamics simulations can determine the thermal stability of materials. Figure S1 plots the molecular dynamic calculation of ScHfCO₂ MXene. The total energies fluctuate near the same energy, which denotes the thermal stability of ScHfCO₂.

3.2 Electronic properties of ScHfCO₂

Fig.3 shows the band structures of Sc₂CO₂ and ScHfCO₂. Sc₂CO₂ is a direct nonmagnetic semiconductor with band gap of 1.85 eV with the conduction band minimum (CBM) and valence band maximum (VBM) all at Γ points, which agrees with the results of references [63, 64]. The substitution of Hf atoms makes the redshift of the VBM, and CBM transfers from Γ point to M point, which induces the decrease of the band gap of ScHfCO₂ and changes from direct semiconductor to indirect semiconductor. ScHfCO₂ is a nonmagnetic semiconductor, its band gap is 0.80 eV.

Fig.3 Band structures of Sc2CO2 (a) and ScHfCO2 (b) monolayers.

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Fig.4 plots the band structures of ScHfCO₂ under strain from +5% to ‒5%. We only show the band structures of ScHfCO₂ under ‒5%, ‒3%, +3% and +5% strains to make the manuscript more clearly. We can see that the effect of biaxial strains on band gap is relatively small. The band gap of ScHfCO₂ under strain from ‒5% to +4% ranges from 0.77 to 0.81 eV [see Fig.5(a)], then has a large increase at +5% strain (0.96 eV). Band gap of ScHfCO₂ without strain is 0.80 eV, and it reduces with the increasing compressive strain and is 0.77 eV at ‒5% strain. The blueshift of CBM makes the band gap increase with the increase of the tensile strain. At +5% strain, the energy at K point larger than the energy at Γ point makes the VBM transfer from Γ point to K point, so ScHfCO₂ changes to M→K indirect semiconductor. Fig.5(b) plots the variation of VBM and CBM of ScHfCO₂ under strain. We note that CBM decreases at M point, VBM increases at Γ point, and ScHfCO₂ under strain from ‒5% to +4% maintains the indirect bandgap characteristics. At +5% strain, VBM transfers to K point, ScHfCO₂ exhibits the character of M→K indirect semiconductor.

Fig.4 Band structures of ScHfCO2 monolayer under biaxial strain.

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Fig.5 Band gap (a) and variation of VBM and CBM (b) of ScHfCO2 under strain.

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Fig.6 shows the projected density of state (PDOS) of ScHfCO₂ under strain. For ScHfCO₂ without strain, CBM mainly comes from Sc-d and Hf-d orbits, and VBM mainly comes from C-p, O-p, Sc-d and Hf-d orbits. The contribution of Sc-d and Hf-d orbits to CBM gradually reduces with the increasing tensile strain, while the contribution of Sc-d and Hf-d to CBM gradually increases and decreases with the increasing compressive strain, respectively. The blueshifts of Sc-d and Hf-d orbits in CBM increase the band gaps of ScHfCO₂ under tensile strains. The slight redshift of Sc-d and Hf-d orbits in CBM induces the reduction of band gaps. At +4% and +5% strains, the contribution of Hf-d and O-p orbitals to VBM drastically increases, while the contribution of Sc-d and C-p orbitals to VBM significantly decreases.

Fig.6 PDOS diagram of ScHfCO2 under 0%, ‒3%, ‒5%, +3%, and +5% strains.

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Figure S2 plots the partial band structures of ScHfCO₂ under strains. It is noted that the contribution of Sc atoms to CBM gradually decreases under strains from ‒5% to +5%, Hf atom at 0% strain has the largest contribution of 37.4% to CBM, and then decreases with the increasing tensile and compressive strains. The contribution of C and O atoms to CBM has no significant change. At +5% strain, Sc and Hf atoms have the contribution of 26.8% and 30.4% to CBM, respectively. The contribution of Sc and O atoms to VBM gradually increases under strains from ‒5% to +3% strain, while the contribution of Hf and C atoms to VBM gradually decreases. At +4% strain, the contribution from Sc and C atoms to VBM drastically decreases to 7.4% and 21.4%, while Hf and O atoms drastically increase the contribution of 14.2% and 24.2% to VBM, respectively.

3.3 Charge density and work function

Fig.7 illustrates the differential charge density of ScHfCO₂ under strain. For ScHfCO₂ under strain, Sc and Hf atoms lose charges, while the surrounding C and O atoms gain charges. Bader charge is performed to quantitatively investigate the charge transfer of each atom. From Fig.7(e), the charges lost by Hf atom and obtained by O atom under strains all slightly increase. The charge of Sc atom under strain from ‒5% to +2% gradually decreases and ranges from ‒1.74e to 1.70e, then increases to ‒1.71e at +5% strain. The charge of C atom under strain from ‒5% to +4% gradually decreases and ranges from 1.24e to 1.18e, which induces in the reduction of C−O bond length.

Fig.7 Differential charge density (a‒d) and Bader charge (e) of ScHfCO2 under strains. Yellow denotes charge accumulation, cyan denotes charge depletion.

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Working function (WF) can measure the required energy to remove an electron from the surface. Fig.8 presents the WFs of ScHfCO₂ under strain. We can see that the WFs of ScHfCO₂ fluctuately decrease and range from 5.2 to 5.9 eV. This indicates that biaxial strain is favorable for ScHfCO₂ to become thermionic electron emission devices. ScHfCO₂ without strain has the WF of 5.7 eV, smaller than those of ZrScCO₂, ZrMnCO₂, and ZrFeCO₂, but larger than those of ZrMCO₂ (M = Ti, V, Cr, W, Ta, Hf, MO, Nb) [65].

Fig.8 Work function of ScHfCO2 MXene under strain.

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Figure S3 plots planar average electrostatic potential (PAEP) of ScHfCO₂ under strain. We can see the PAEP oscillates in entire supercell, each valley denotes the position of an atomic layer. Left and right gray denote ScO and HfO layers. Middle cyan demotes C layer. Fig.9 presents electrostatic potential energy Φ and its difference ΔΦ between ScO and HfO layers of ScHfCO₂ under strain. The appearance of built-in electric field in ScHfCO₂ under strain enhances charge redistributes across Janus layer. HfO layer has higher electrostatic potential energy than ScO layer, so the photogenerated electron can easily flow from HfO to ScO layer. The ΔΦ gradually decreases in the strain from 0 to +2% and ranges from 0.52 to 0.04 eV. When the strain is larger than +2% strain, the photogenerated electron flows from ScO to HfO layer because of the charge redistribution, ΔΦ is negative and ranges from ‒0.24 to ‒1.22 eV in the strain from +2% to +5%. The ΔΦ under compressive strain fluctuately decreases and ranges from 1.52 to 0.52 eV.

Fig.9 The electrostatic potential energy Φ (a) and its difference ΔΦ (b) of ScHfCO2 under strain.

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3.4 Optical properties

The electronic transitions between different energy levels can cause the spectrum, especially the band structure and DOS are closely related with the imaginary peak of dielectric function $\epsilon (\omega )$, which is equal to ${\epsilon }_1(\omega )+\mathrm{i}{\epsilon }_2(\omega )$, ${\epsilon }_1(\omega )$ and ${\epsilon }_{\mathrm{s}}(\omega )$ mean the real part and imaginary part of $\epsilon (\omega )$, respectively. Absorption coefficient $I(\omega )$ is obtained by

(8)$I(\omega )=\sqrt{2}\omega [\sqrt{{\epsilon }_1^2(\omega )+{\epsilon }_2^2(\omega )}-{\epsilon }_1(\omega ){]}^{1/2}.$

Fig.10(a)‒(d) show the ${\epsilon }_1(\omega )$ and ${\epsilon }_2(\omega )$ of dielectric function $\epsilon (\omega )$ of ScHfCO₂ under strain. The ${\epsilon }_2(\omega )$ denotes the possible inter band transition and direct semiconductors have most likely transition. The first absorption peak of ${\epsilon }_2(\omega )$ for ScHfCO₂ under tensile strain locates at about 2.97 eV and is in visible region. It ranges from 1.40 to 2.18. The first absorption peak of ${\epsilon }_2(\omega )$ under compressive strain also locates in visible region and ranges from 1.33 to 1.52. We note that the intensity of transition peak under compressive strain is higher than those under tensile strain.

Fig.10 The dielectric constant (a‒d) and absorption coefficient (e, f) of ScHfCO2 under strain.

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For ${\epsilon }_1(\omega )$, static dielectric constant ${\epsilon }_1(0)$ (dielectric constants at 0 eV) reflects the electrical conductivity of materials. Larger static dielectric constant means better conductivity. ${\epsilon }_1(0)$ of ScHfCO₂ without strain is 2.1, and ranges from 2.10 to 2.19 under tensile strain. ${\epsilon }_1(0)$ of ScHfCO₂ under compressive strain ranges from 2.10 to 2.27, which are generally larger than those under tensile strain. This means that ScHfCO₂ under compressive strain has better conductivity than ScHfCO₂ under tensile strain. ScHfCO₂ under ‒3% strain has the biggest ${\epsilon }_1(0)$, indicating that the system has the best conductivity.

Absorption coefficient $I(\omega )$ is closely related with the ${\epsilon }_2(\omega )$, and higher energy of absorption peak is from the inter band transition [66]. The absorption edge of ScHfCO₂ under strain is approximately equal to its bandgap, so I of ScHfCO₂ is zero in infrared region (Energy (E) < 1.64 eV). From Fig.10(e) and (f), I increases with the increasing tensile strain in visible region (1.64 < E < 3.19) and the top I is 10.11%. We note that I under compressive strain in visible region is lower than those under tensile strain, and the top I under compressive stain is 6.0%. The main absorption range of ScHfCO₂ under strain is in ultraviolet region, and the absorption intensity of ScHfCO₂ under compressive strain is higher than those under tensile strain. Therefore, ScHfCO₂ under compressive strain has potential photoelectric conversion switch in ultraviolet region.

3.5 Quantum capacitance

Fig.11 presents the C_diff and Q at different potential. Here, we consider two voltage ranges, one is from ‒0.6 to 0.6 V (aqueous system), the other is from ‒1.2 to 1.2 V (ionic/organic system). From Fig.11(a) and (b), the C_diff under strain at positive bias are all very small at aqueous system. The C_diff at 0 V ranges from 0.02 to 25.67 μF/cm². At negative bias, the C_diff of ScHfCO₂ without strain fluctuates and reaches its top value (C_c) of 868.58 μF/cm². Applying tensile strain improves the C_c of ScHfCO₂, with the largest C_c of 2260.87 μF/cm² at +5% strain. The C_c values under different compressive strains are all smaller than those under tensile strains, and range from 431.11 to 904.12 μF/cm². Large voltage drastically increases the top C_diff (C_a) under strain at positive bias. In addition, large voltage has little effect on the C_c under strains of +2%, +4%, +5%, but sharply improves the C_c under compressive strain.

Fig.11 The Cdiff (a, b) and Q (c, d) of ScHfCO2 monolayer under biaxial strains.

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From Fig.11(c) and (d), in narrow voltage, the Q_s at positive bias are all very small, with the maximum Q (Q_a) of 1.37 μC/cm² at ‒2% strain. The top Q (Q_c) at negative bias are much larger than Q_a for ScHfCO₂ under strain, which ranges from ‒47.42 to ‒262.87 μC/cm². Large voltage greatly increases the Q_a and Q_c of all systems.

|Q_a|/|Q_c| or |C_a|/|C_c| is often used to predict the electrode type of materials. Here, |Q_a|/|Q_c| is selected in our work. |Q_a|/|Q_c| larger than 1 denotes that the system is anode material, while |Q_a|/|Q_c| smaller than 1 indicates the system is cathode material. Tab.2 lists the Q_a, Q_c, and |Q_a|/|Q_c| of ScHfCO₂ under strains. We can see that all |Q_a|/|Q_c| are all much smaller than 1 in aqueous system, indicating that ScHfCO₂ under strain are promising cathode materials, which is same as ZrHfCO₂ [65]. Larger voltage improves the character of cathode materials because of their much larger |Q_c| when compared with those in aqueous system.

Tab.2 Qa, Qc, and |Qa|/|Qc| of ScHfCO2 under strains. All systems are cathode materials.

System	Aqueous electrolytes				Ionic/organic electrolytes
	|Qa|	|Qc|	|Qa|/|Qc|		|Qa|	|Qc|	|Qa|/|Qc|
−5%	0.41	94.77	0.0043		61.22	415.61	0.15
−4%	0.76	65.89	0.0116		67.00	367.57	0.18
−3%	0.03	47.42	0.0006		73.20	351.53	0.21
−2%	1.37	121.99	0.0113		53.71	363.93	0.15
−1%	0.03	111.81	0.0002		63.66	380.79	0.17
0%	0.52	203.86	0.0026		43.86	430.61	0.10
+1%	0.02	128.51	0.0002		58.31	327.06	0.18
+2%	0.48	204.22	0.0024		42.79	387.60	0.11
+3%	0.03	151.67	0.0002		60.96	401.31	0.15
+4%	0.04	210.33	0.0002		63.86	404.06	0.16
+5%	0.03	262.87	0.0001		27.48	494.40	0.06

4 Conclusion

We investigated the electronic properties and C_diff of ScHfCO₂ under biaxial strain. The stability of ScHfCO₂ under biaxial strain is confirmed by binding energy and molecular dynamic simulation. Sc₂CO₂ is a direct nonmagnetic semiconductor, while Hf substitution makes the system change into nonmagnetic indirect semiconductor. ScHfCO₂ under strains all maintains the indirect bandgap characteristics. Biaxial strain is favorable for ScHfCO₂ to become thermionic electron emission devices. HfO layer has higher electrostatic potential energy than ScO layer, so the photogenerated electron can easily flow from HfO to ScO layer, which forms the built-in electric field. ScHfCO₂ under compressive strain are potential photoelectric conversion switch in ultraviolet region. Large voltage has little effect on the C_c under strains of +2%, +4%, +5%, but sharply improves the C_c under compressive strain. ScHfCO₂ under strains are cathode materials in whole voltage.

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The authors declare no competing interests and no conflicts.

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