Exceptional Optical Absorption of Buckled Arsenene Covering a Broad Spectral Range by Molecular Doping.

Using density functional theory calculations, we demonstrate that the electronic and optical properties of a buckled arsenene monolayer can be tuned by molecular doping. Effective p-type doping of arsenene can be realized by adsorption of tetracyanoethylene and tetracyanoquinodimethane (TCNQ) molecules, while n-doped arsenene can be obtained by adsorption of tetrathiafulvalene molecules. Moreover, owing to the charge redistribution, a dipole moment is formed between each organic molecule and arsenene, and this dipole moment can significantly tune the work function of arsenene to values within a wide range of 3.99-5.57 eV. Adsorption of TCNQ molecules on pristine arsenene can significantly improve the latter's optical absorption in a broad (visible to near-infrared) spectral range. According to the AM 1.5 solar spectrum, two-fold enhancement is attained in the efficiency of solar-energy utilization, which can lead to great opportunities for the use of TCNQ-arsenene in renewable energy. Our work clearly demonstrates the key role of molecular doping in the application of arsenene in electronic and optoelectronic components, renewable energy, and laser protection.

Introduction

Phosphorene, a monolayer composed of puckered structures of phosphorus atoms, has attracted much attention in recent years owing to its remarkable electronic,[1,2] thermal,[3−5] mechanical,[6] and optical[7] properties. Its great potential in nanoelectronics,[8−10] spintronics,[11] sensors,[12] and energy conversion and storage[13−15] has been addressed. The fascinating properties and wide application of phosphorene have driven the search for monolayers of other group V elements. Recently, Zhang et al.[16] predicted a new two-dimensional (2D) semiconducting material named buckled arsenene. It is an intrinsic semiconductor with a sizable band gap[16−20] and ultrahigh mobility.[19,20] Its band gap (larger than 2 eV) makes it useful for transistors with a high on/off current ratio and optoelectronic devices working under blue or ultraviolet light.[16] Moreover, its stability has been demonstrated by calculations of its phonon spectrum[16−19] and ab initio molecular dynamics calculations.[18] Very recently, Tsai et al.[21] successfully synthesized multilayered arsenene nanoribbons on InAs substrates by using a plasma-assisted process. All these investigations indicate that buckled arsenene can be a promising 2D semiconducting material in various fields.

Owing to the atomic thickness and large surface-to-volume ratio of 2D materials, molecular doping has been widely adopted to enhance their conductivity, reduce their carrier-injection barrier, and control their band gap to promote their application in electronics. For example, graphene is regarded as an ideal material for hosting the molecules.[22−33] By adsorbing various types of organic molecules on graphene, the type and density of charge carriers can be controlled, which is vital for different practical applications. Solís-Fernández et al.[28] found that the doping of graphene can be varied from p- to n-type by gradually increasing the concentration of piperidine. Thus, p–n junctions can be formed on a graphene sheet by controlling the coverage of organic molecules. Recently, the interaction between organic molecules and 2D semiconducting materials such as MoS2[34−38] and black phosphorene[39,40] has also been the subject of many studies. These works showed that molecular doping not only changes the electronic properties of 2D materials, but also modifies their optical[34−37] and thermoelectric[38] properties. For instance, Mouri et al.[34] found that the photoluminescence intensity of MoS2 can be remarkably improved by chemical doping with p-type molecules.

On the basis of these developments, we expect molecular doping to be a powerful tool for tuning the electronic and optical properties of buckled arsenene. In principle, an arsenic atom in buckled arsenene forms three σ-bonds with neighboring arsenic atoms and leaves a lone pair of electrons. A previous study indicated that such lone pairs of electrons can enhance the surface interaction between black phosphorene and dopant molecules.[41] Therefore, buckled arsenene is expected to be highly suitable for hosting organic molecules owing to the presence of lone pairs of electrons. However, the effects of molecular doping on the electronic and optical properties of buckled arsenene are still not well understood.

In this paper, we investigated the impact of molecular doping on the electronic and optical properties of buckled arsenene by performing first-principles calculations. We considered three representative organic molecules—tetracyanoethylene (TCNE), tetracyanoquinodimethane (TCNQ), and tetrathiafulvalene (TTF), which have been extensively investigated in organic chemistry and widely used in fabrication of electronic devices.[42−47] We found that surface doping of arsenene with organic TCNE and TCNQ can lead to effective p-doping. Meanwhile, n-doped arsenene can be obtained by adsorption of TTF. For the TCNE-doping of arsenene, because the acceptor level of TCNE is just 0.082 eV above the valance band edge of arsenene, room temperature was sufficient to thermally ionize the surface dopants. We also found that beyond enhancing the electronic properties, TCNQ doubled the optical absorption ability of arsenene in the visible and near-infrared spectral regions. The effective doping and enhancement of optical absorption are expected to result in good performance of TCNQ–arsenene in both nanoelectronics and solar-energy harvesting.

Results and Discussion

Adsorption Configurations

For each molecule, we considered several adsorption configurations. To compare their stability, we calculated their adsorption energy (Ead) on arsenene as followswhere Emolecule, Earsenene, and Emolecule+arsenene represent the energies of the dopant molecule, pristine arsenene, and molecule-doped arsenene, respectively.

We first explored the adsorption of each molecule on arsenene by identifying the most energetically favorable configurations. Various high symmetry adsorption sites for these molecules are summarized in Figure S1, and the most favorable adsorption configurations are shown in Figure . Adsorption on other sites is characterized by lower adsorption energy. Hereafter, all of the results and discussions are related to the most energetically favorable configurations. For the TCNE molecule, all the cyano groups align parallel to the armchair direction of arsenene, with an Ead of 0.54 eV and an adsorption height (h) of 3.23 Å. For the TCNQ molecule, the benzene ring is located right above an arsenic atom in the upper plane, allowing high doping efficiency and a rather stable structure (Ead = 0.84 eV; h = 3.31 Å). For the TTF molecule, the two C3S2 rings are also located above the arsenic atoms of arsenene. In addition, the TTF molecule bends over the arsenene sheet, resulting in an Ead of 0.84 eV and an h of 3.08 Å.

Figure 1

Top and side views of the most stable configurations of TCNE, TCNQ, and TTF molecules adsorbed on buckled arsenene. The orange and blue sticks represent the As atoms in the upper and lower planes, respectively, of arsenene. The black, red, yellow, and white spheres represent C, N, S, and H atoms, respectively.

Effects of Doping on the Electronic Structure of Arsenene

Figure a–c shows the isosurface of charge-density difference after the adsorption of TCNE, TCNQ, and TTF on the arsenene monolayer, respectively. The pink and white regions denote the accumulation and depletion of electrons, respectively. For the systems with adsorbed TCNE and TCNQ, the electrons are transferred from the arsenene layer to the molecules, and both the TCNE and TCNQ molecules act as electron acceptors, primarily owing to their high adiabatic electron affinity (2.884 eV for TCNE[48] and 2.80 eV for TCNQ[49]). The electrons donated by the arsenene layer are mainly located in the cyano groups of the molecules and the interlayer region between the molecules and arsenene sheet, suggesting that the cyano groups are the electron-accepting groups. For the TTF–arsenene adsorption system, the white regions around the TTF molecule indicate that it acts as an electron donor because of its low ionization potential of 6.83 eV.[50] The transferred electrons are mainly distributed in arsenic atoms in the contact region of the arsenene host layer. The more accurate Bader analysis[51−53] clearly shows that 0.22, 0.20, and −0.03 electrons are transferred from the arsenene layer to TCNE, TCNQ, and TTF molecules, respectively. Therefore, the adsorption of one organic molecule (density is 1.11 × 1014 cm–2) induces carrier injection in arsenene with concentrations of 2.44 × 1013, 2.22 × 1013, and 3.33 × 1012 cm–2, respectively, which can significantly enhance the performance of arsenene in nanoelectronics. In practical application, the carrier concentration will change with adsorption density. Moreover, owing to the charge redistribution, a dipole moment forms between the organic molecule and arsenene, and it can significantly change the work function of arsenene. As shown in Figure d, the adsorption of acceptor molecules (TCNE or TCNQ) results in an increase in the work function: the work functions of TCNE–arsenene and TCNQ–arsenene are 5.57 and 5.40 eV, respectively. In contrast, the donor molecules (TTF) lead to a considerable reduction in the work function from that of pristine arsenene (5.16 eV) to TTF–arsenene (3.99 eV). This large tunable range of work function from 3.99 to 5.57 eV suggests the great potential of molecular doping in arsenene-based nanoelectronics. It is worth mentioning that the charge transfer is much smaller for the TTF–arsenene systems, while the change in work function is much larger than that in the TCNE–arsenene and TCNQ–arsenene systems. Actually, the relationship between change of work function and the charge transfer is not linear in most situations. This is because the change of work function not only depends on the shift of the Fermi level but also depends on the dipole moment, and the relationship between these two items and the charge transfer are nonlinear. (Please refer to the Supporting Information).

Figure 2

Isosurface of charge-density difference with an isovalue of 0.0002 e Å–3 for (a) TCNE–arsenene, (b) TCNQ–arsenene, and (c) TTF–arsenene adsorption systems. The top and side views are shown in the upper and lower panels, respectively. The blue, black, red, yellow, and white spheres represent As, C, N, S, and H atoms, respectively. The pink and white regions denote the accumulation and depletion of electrons, respectively. (d) Work functions of pristine arsenene, TCNE–arsenene, TCNQ–arsenene, and TTF–arsenene systems.

Work function is one of the critical properties that must be evaluated for the application of 2D materials in electronic devices. It is well-known that materials with low work function can potentially be applied in field-emission devices. Several strategies were employed in previous studies to decrease the work function of nanomaterials. For example, the work function of boron-doped carbon nanotubes was found to be 1.7 eV lower than that of pristine carbon nanotubes.[54] The adsorption of alkali metals on carbon nanotubes[55] and graphene[56] was also found to significantly lower their work functions. In our study, by adsorbing the TTF molecule on the surface of buckled arsenene, the work function can be decreased by up to 1.17 eV. Moreover, this method has the advantage of not requiring the production of vacancies in the host to anchor the impurities and also avoiding the issue of clustering of alkali metal atoms. All in all, this is a feasible and controllable method for reducing the work function of arsenene.

To assess the effect of molecular doping on the electronic properties of arsenene, we calculated the projected band structure of pristine arsenene and the TCNE–arsenene, TCNQ–arsenene, and TTF–arsenene adsorption systems; the calculated structures are shown in Figure . It can be clearly seen that pristine arsenene is an indirect semiconductor with a band gap of 1.59 eV (Figure a), which is in good agreement with a previously reported result.[17] Compared with pristine arsenene, a flat band, which is dominated by the lowest unoccupied molecular orbital (LUMO) of organic molecules, appears near the Fermi level in the TCNE–arsenene and TCNQ–arsenene systems (Figure b,c). The impurity bands are 0.041 and 0.077 eV above the Fermi level, representing the formation of acceptor states. These empty levels could accept excited electrons and produce holes in the arsenene sheet, signifying p-type doping of arsenene. For the TTF–arsenene system, we found a flat band induced by the highest occupied molecular orbital (HOMO) of the TTF molecule at 0.061 eV below the Fermi level, which suggests that donor states are formed. As a result, the adsorption of TTF on arsenene results in n-type doping behavior. Beyond the new doping level, the main features of the band structure are not changed, and the stability of arsenene remains intact without structural breakage.

Figure 3

Projected band structures of (a) pristine arsenene, (b) TCNE–arsenene, (c) TCNQ–arsenene, and (d) TTF–arsenene systems. The black and red symbols represent the contribution of the arsenene layer and organic molecules, respectively. The Fermi level was set to zero and is indicated by the black dashed line.

To gain deeper insight into the electronic properties of arsenene systems with adsorbed organic molecules, we calculated the partial charge densities of the impurity band for the TCNE–arsenene, TCNQ–arsenene, and TTF–arsenene systems; the results are shown in Figure . It can be seen that the impurity band of all the adsorbed systems is dominated by the organic molecules. For TCNE and TCNQ, the profiles of the partial charge densities are similar to those of their respective LUMO (Figure S2a,b). Meanwhile, for TTF, the profile of the partial charge density is quite like that of its HOMO (Figure S2c). Therefore, the excited electrons and holes are spatially separated, which leads to good characteristics for optoelectronic applications.

Figure 4

Partial charge densities of impurity band of (a) TCNE–, (b) TCNQ–, and (c) TTF–arsenene systems. The blue, black, red, yellow, and white spheres represent As, C, N, S, and H atoms, respectively. The isosurface value was set to 0.002 e Å–3.

Semiconductor p–n junctions are essential building blocks of today’s electronic and optoelectronic devices. With the rapid development of 2D materials, they are now considered as promising material candidates for post-silicon electronics. Recently, 2D-material-based p–n junctions have attracted wide interest because of their great potential in various applications such as solar cells,[57,58] diodes,[59,60] and photodetectors.[61] To construct p–n junctions, the key is to precisely control the type of charge carriers in 2D materials as either p-type or n-type. In this work, the doping gaps for TCNE–, TCNQ–, and TTF–arsenene systems are 0.082, 0.147, and 0.655 eV, respectively. Thus, both the TCNE and TCNQ molecules can introduce shallow acceptor states in the band gap of arsenene close to the valence band edge, while the deep donor states can be introduced in the band gap after the adsorption of the TTF molecule. The DFT calculation results show that the doping gap of TCNE–arsenene is close to the thermal active energy (kBT = 0.026 eV at 300 K), which suggests that the thermal ionization of surface dopants can happen near room temperature. Moreover, the doping gap and doping concentration can be further controlled by applying an out-of-plane electric field and in-plane strain.[39] Thus, the carrier type in arsenene can be effectively tuned to p-type or n-type by molecular doping, which can be used to construct arsenene p–n junctions, as revealed recently by Gao et al.[62]

Effect of Molecular Doping on Optical Absorption

Next, we investigated the effect of molecular doping on the optical properties of arsenene. As a benchmark, we calculated the imaginary parts of the dielectric functions for a pristine TCNQ molecule by using both Perdew–Burke–Ernzerhof (PBE) and HSE06 functionals, as shown in Figure S3. We selected the diagonal parts (ε2) as an example. The PBE and HSE06 methods each predicted a high adsorption peak at approximately 878 and 615 nm, respectively. The results predicted by the HSE06 method are in good agreement with the available experimental data (about 580–650 nm with different cations).[63] Hereafter, all the presented results were calculated using the HSE06 method. Figure shows the computed results of the imaginary parts of the dielectric functions for pristine arsenene and the adsorption systems. For pristine arsenene, many absorption peaks can be found in the ultraviolet region (<400 nm), but much weaker absorption is seen in the visible region (400–700 nm), which are consistent with previous results.[64,65] For the TCNE–arsenene and TTF–arsenene systems, the imaginary parts of their dielectric functions are rather similar to that of the dielectric function of pristine arsenene. Interestingly, for TCNQ–arsenene, two peaks appear in the visible region, revealing high optical absorption as a consequence of molecular doping. Although the charge transfer is similar, the band alignment of TCNE–arsenene (Figure b) and TCNQ–arsenene (Figure c) is different because the difference in the initial position of LUMO of TCNE/TCNQ. As a result, the optical absorption of TCNE–arsenene and TCNQ–arsenene is different.

Figure 5

Imaginary parts of dielectric functions of pristine arsenene and TCNE–arsenene, TCNQ–arsenene, and TTF–arsenene systems calculated by HSE06 functional.

Figure shows the optical absorption spectra of the pristine arsenene and TCNQ–arsenene systems. The absorption coefficient was obtained from the following equation[66]where ε12(ω) and ε22(ω) are the real and imaginary parts, respectively, of the dielectric constant. For pristine arsenene, many absorption peaks with intensity above 1.0 × 105 cm–1 can be found in the ultraviolet region. According to the AM 1.5 solar spectrum,[67] visible and near-infrared radiation accounts for most of the solar energy arriving at the surface of the earth. Therefore, a frequently employed strategy to obtain high-efficiency photovoltaic devices is to increase the optical absorption in the visible and near-infrared regions. Obviously, pristine arsenene is not a promising candidate for photovoltaics because of its poor absorption ability in the visible and near-infrared regions. Here, we propose that the adsorption of TCNQ molecule is a feasible approach to enhance the optical-absorption ability of arsenene in the major range of the solar spectrum. The absorption peaks in the ultraviolet region are reduced slightly after the adsorption of TCNQ (Figure ), and a new absorption peak with an intensity of about 1.90 × 104 cm–1 can be found at approximately 610 nm in the visible region. In addition, the absorption spectrum of TCNQ–arsenene is much broader (in a wide range from the ultraviolet region to the near-infrared region) than that of pristine arsenene. In fact, the optical absorption spectrum of TCNQ–arsenene almost overlaps the entire incident solar spectrum, thus suggesting that this adsorption system is a highly efficient absorber of light in the visible and near-infrared range. We can quantitatively estimate the solar absorption ability (η) of a material from the following equationwhere λ is the wavelength of the incident light, a(λ) is the absorption spectrum, and S(λ) is the incident AM 1.5G solar flux. The η of TCNQ–arsenene is 27.97 W/cm3, which is about twice that of pristine arsenene (14.24 W/cm3). Thus, it can be concluded that the TCNQ–arsenene system is a high-efficiency light-absorbing material, which is promising for application in devices for harvesting of solar energy.

Figure 6

Optical-absorption coefficient of pristine arsenene and TCNQ–arsenene systems calculated by HSE06 functional. The range of light absorption by TCNQ–arsenene system overlaps the whole wavelength range of the incident AM 1.5G solar flux.

Laser technology has been widely used in both military and civilian applications. For example, a YAG laser (wavelength: 1064 nm) is used in many important applications such as material cutting[68] and drug delivery.[69] However, an earlier investigation showed that the picosecond pulses of Nd:YAG laser irradiation can cause ocular damage.[70] The protection of humans from unwanted laser irradiation is thus very important for wide adoption of laser technology, and the search for materials that can protect them from unwanted incident laser beams is obviously the key. Pristine arsenene can be used as a protective material for laser irradiation at wavelengths below about 500 nm. On the other hand, TCNQ–arsenene offers protection from laser beams in the ultraviolet, visible, and near-infrared regions. Moreover, the absorption intensity of TCNQ–arsenene is stronger than that of pristine arsenene. For example, the TCNQ–arsenene system’s absorption intensity of laser irradiation from a YAG laser source is about eight times stronger than that of pristine arsenene. Thus, TCNQ–arsenene is also a promising protective material against unwanted laser irradiation.

Conclusions

We performed density functional theory calculations to study the effects of molecular doping on the electronic and optical properties of buckled arsenene. TCNE and TCNQ were found to be efficient p-type dopants, while TTF was found to be an n-type dopant. Our results indicate that arsenene-based p–n junctions can be manufactured by doping arsenene with these organic molecules. In addition, the adsorption of the TTF molecule can significantly reduce the work function of arsenene. Interestingly, the ability of arsenene to absorb solar energy is doubled owing to the adsorption of TCNQ molecules on its surface. Therefore, the TCNQ–arsenene system is a promising material for application in solar-energy harvesting. Our work provides a molecular-level mechanism for tuning the electronic and optical properties of arsenene and unveils its potential for application in nanoelectronics and optoelectronic devices.

Computational Details

First-principles calculations were performed by using the plane-wave-based Vienna ab initio simulation package.[71] The interactions between the ions and valence electrons were treated by the projected augmented-wave method.[72] We used the generalized gradient approximation with the PBE form for the exchange–correlation functional.[73] Because the PBE functional is less reliable in describing the optical properties of semiconductors, the hybrid Heyd–Scuseria–Ernzerhof (HSE06) functional[74] was also selected to compute the optical properties. The zero-damping vdW-D3 correction proposed by Grimme[75] was used to describe the long-range interaction between organic molecules and the arsenene layer. The kinetic-energy cutoff of 400 eV and 7 × 7 × 1 Monkhorst–Pack[76]k-point meshes were used. We adopted a 4 × 4 × 1 supercell containing 32 As atoms to mimic the adsorption of organic molecules on the arsenene monolayer. Moreover, to avoid interactions between periodic structures, a large vacuum region of 20 Å was adopted. The geometry was optimized until the total energy converged to 10–5 eV and the forces of all atoms are smaller than 0.01 eV/Å.