Electronic and Near-Infrared-II Optical Properties of I-Doped Monolayer MoTe2: A First-Principles Study.

Near-infrared-II (NIR-II, 1000-1700 nm) fluorescence imaging is widely used for in vivo biological imaging. With the unique electronic structures and capability of band-gap engineering, two-dimensional (2D) materials can be potential candidates for NIR-II imaging. Herein, a theoretical investigation of the electronic structure and optical properties of iodine (I)-doped monolayer MoTe2 systems with different doping concentrations is carried out through simulations to explore their NIR optical properties. The results suggest that the emergence of impurity levels due to I doping effectively reduces the bandwidth of I-doped monolayer MoTe2 systems, and the bandwidth decreases with the increase in the I doping concentration. Although the I and Mo atoms possess clear covalent-bonding features according to the charge density difference, impurity levels induced by the strong hybridization between the I 5p and Mo 4d orbitals cross the Fermi level, making the doped systems exhibit metallic behavior. In addition, with the increase in the I doping concentration, the energy required for electron transition from valence bands to impurity levels gradually decreases, which can be linked to the enhancement of the optical absorption in the red-shifted NIR-II region. Meanwhile, with a higher I doping concentration, the emission spectra, which are the product of the absorption spectra and quasi-Fermi distributions for electrons and holes, can be enhanced in the NIR-II window.

Introduction

In recent years, near-infrared fluorescence imaging technology has played an important role in applications such as biological imaging,[1−4] drug distribution tracking,[5,6] disease diagnosis,[7,8] and tumor imaging.[9,10] Compared with routine imaging technologies including computed tomography (CT),[11] positron emission tomography (PET),[12] single-photon emission computed tomography (SPECT),[13] magnetic resonance imaging (MRI),[14] and ultrasound imaging,[15] the greatest advantage of NIR fluorescence imaging is its high spatial resolution due to the reduced photon scattering, light absorption, and autofluorescence background.[16] In general, photon scattering intensity decays exponentially with increasing wavelengths; compared with visible light (400–700 nm) and near-infrared I (NIR-I, 700–900 nm), NIR-II (1000–1700 nm) can provide the highest spatial-temporal resolution with the lowest autofluorescence and deepest penetration, which has attracted much attention in the potential diagnosis.[17−19]

To date, materials such as carbon nanotubes,[20,21] quantum dots,[22,23] rare earth,[24,25] gold clusters,[9,26] polymers,[27] and small molecules[28,29] have exhibited excellent NIR-II fluorescence. Rare earth nanoparticles represented by lanthanide compounds, as the new generation of NIR fluorescent nanoprobes, have recently aroused enormous interest in biological imaging applications owing to their unique properties of low photobleaching, long luminescence lifetime, and narrow emission width.[30−32] However, lanthanide compounds may not be the answer to NIR-II imaging, considering their relatively low quantum yield and it is still important to explore other types of materials. The wavelength of NIR-II (1000–1700 nm) corresponds to 0.73–1.24 eV, which leads to the natural band gap of 2D materials. Band-gap engineering is a powerful technique to adjust the properties of 2D materials.[33,34] Accordingly, continuous efforts such as doping have been devoted to finding semiconductor materials with appropriate optical properties, which are indicated by their band gaps.

As an important member of 2D transition metal dichalcogenides (TMDs) with unique optical and electronic properties, MoTe2 is an indirect band-gap semiconductor with an optical gap of approximately 1.0 eV.[35−37] As the thickness of bulk MoTe2 reduced to a monolayer (1.1 eV), some special phenomena such as the mutual conversion between the indirect and direct band gap can be observed,[38,39] which provides an attractive direction for adjusting the properties of materials on demand. For example, Kanoun et al. demonstrated that transition metal atoms such as V-, Cr-, Mn-, Fe-, and Co-doped monolayer MoTe2 can induce magnetic moment due to strong p–d hybridization between 5p states of Te atoms and 4d states of Mo atoms.[40] In addition, 3d and 4d transition metal atom-doped MoTe2 has been studied by Kumar et al., indicating that doping can be used to tune the phase transition in MoTe2.[41] In the research presented by Liu et al., the impurity level induced by the strong hybridization between the Te 5p and Au 5d orbitals crosses the Fermi level, making the Au-doped monolayer MoTe2 system exhibit metallic behavior with 0 band gap.[42]

At present, researchers have done some research on monolayer MoTe2 and achieved some research results, but there are still few studies on the optical properties of monolayer MoTe2, especially in the NIR-II. Considering the potential NIR-II applications and band-gap engineering, the halogen I atom is an excellent candidate dopant to improve the electronic and optical properties of materials. Recently, Tang et al. simulated the enhancement of visible light absorption of pristine BiOCl by band-gap engineering (the substitutional I doping at the Cl site) due to the electronegativity difference between Cl and I atoms.[43] Kutlu et al. found that the band structure of the I atom-doped monolayer black phosphorus (BP) system was spin-dependent, where the source of the spin state was mainly the p orbital of the I atom. Moreover, the absorption of low-energy regions was improved to some extent after doping.[44] Wang et al. reported that I doping induced a slight band-gap reduction in the 2D metal-phthalocyanine-based pyrazine-linked conjugated covalent organic framework (c-COF ZnPc-pz) system and improved its absorption intensity in the NIR region, which was attributed to the interaction between the electron donor (ZnPc-pz) and acceptor (I2) in ZnPc-pz-I2.[45]

In this work, the electronic and optical properties of I-doped MoTe2 systems with different I concentrations were systematically studied using first-principles simulations. The results show that I incorporation leads to bandwidth reduction. Meanwhile, n-type doping is identified in the doped systems exhibiting metallic behavior. The optical absorption capacity of the doped systems in the NIR-II window was significantly improved with an increase in the I doping concentration, which indicated that the substitutional I doping can optimize the electronic structure and broaden the potential application fields of MoTe2.

Simulation Details

The electronic and optical properties of pure and I-doped monolayer MoTe2 have been investigated with the Vienna Ab initio Simulation Package (VASP) code, which is based on the framework of density functional theory (DFT).[46−48] The Perdew–Burke–Ernzerhof (PBE) functional, one of the most famous generalized gradient approximation (GGA) functionals is employed to describe the exchange–correlation potential.[49] The projector augmented wave (PAW)[50,51] is utilized to describe the electron–ion interactions with 4d55s1 and 5s25p4 as the valence-electron configurations of Mo and Te atoms, respectively. The calculations are carried out at a constant volume, and the adopted monolayer structure is a 3 × 3 × 1 MoTe2 supercell containing 9 Mo atoms and 18 Te atoms, which is sufficient for the simulations: binding energies of different substituted configurations for doped MoTe2 with 3 × 3 × 1 and relatively large 4 × 4 × 1 supercells are converged (see Table S1 for details). Binding energies of each substitution doping configuration are calculated according to the following expressionwhere Edoped and Epure are the total energies of I-doped and pure monolayer MoTe2, respectively, m is the number of the removed Te atoms, n is the number of I atoms that are doped in the system, and ETe and EI are energy per atom for the Te and I dopant atoms, respectively.

To avoid the interaction between layers due to periodicity and improve the reliability and accuracy of calculations, a vacuum distance of 20 Å is taken along the z-direction perpendicular to the monolayer surface. In this paper, a substitution doping method is performed to replace Te atoms in monolayer MoTe2 with one, two, and three I atoms. To ensure the convergence of calculations, the cutoff energy for the plane-wave basis is set to 450 eV. In addition, all Brillouin zone integrations are performed according to the Monkhorst Pack scheme,[52] a 5 × 5 × 1 k-mesh based on the Γ-centered scheme for relaxation calculations. For structural relaxation, the convergence criteria for energy and force are set to 1 × 10–6 eV and −0.01 eV/Å, respectively. In this work, the Heyd–Scuseria–Ernzerhof (HSE06)[53] hybrid functional is also employed to verify the impact of HF exchange on the band gap and optical properties. The exchange–correlation energy in HSE06 is then defined aswhere ExHF,SR is the short-range HF exchange, ExPBE,SR and ExPBE,LR are the short-range and long-range components of the PBE exchange functional obtained by integration of the model PBE exchange hole,[54,55] and EcPBE is the correlation energy. In the HSE calculations, the mixing parameter is set to the standard value of α = 0.25 and the HF screening parameter μ is set to 0.2 Å–1.

In the random phase approximation (RPA),[56] local field effects are neglected. The optical properties of semiconductors in the linear response range are generally described by the dielectric function ε(ω)where ε1(ω) and ε2(ω) are the real and imaginary parts of the dielectric function, respectively, and ω is the frequency of electromagnetic waves. The imaginary part of the dielectric function ε2(ω) reflects the absorption of photons by the system, which is calculated by the transition of valence electrons between occupied and unoccupied orbits.[57]

Results and Discussion

To set up valid models of doped monolayer MoTe2, different possibilities are explored and evaluated. According to different stacking modes, monolayer MoTe2 has three crystal phases, namely the semiconducting 2H phase, the metallic 1T phase, and the semimetallic distorted octahedral 1T′ phase, respectively.[58−60] The 2H phase, which is commonly studied is the most stable form among the three crystal structures.[61] Monolayer MoTe2 is composed of hexagonally arranged Mo atoms that are sandwiched between two layers of Te atoms with schematic illustrations of top and side views shown in Figure a,b, respectively. According to the substitutional doping method, Te atoms can be replaced by I atoms in MoTe2. All configurations of MoTe2 systems doped with one, two, and three I atoms per supercell are shown in Figure c,f,h, respectively. Binding energies and ground-state energies are used to screen the unphysical configurations. Simulation results show that the two I-doped MoTe2-2 and three I-doped MoTe2-1 exhibit the lowest ground-state energies and binding energies (Figure g,i). Thus, the two configurations are focused on in the simulations.

Figure 1

(a) Top and (b) side views of the monolayer MoTe2. (c) Atomic structure of the single I atom doped with monolayer MoTe2. (d) Schematic diagram of the distance between Te and Mo atoms (dTe–Mo) and the angle of the Te–Mo bond (∠Te–Mo–Te). (e) Schematic diagram of the distance between Mo and I atoms (dMo–I) and the angle of the Mo–I bond (∠Mo–I–Mo). The pink, yellow, and purple balls represent Mo, Te, and I atoms. The top view of six different configurations of MoTe2 systems doped with (f) two and (h) three I atoms, respectively. Comparison of the ground-state energies and binding energies for (g) two and (i) three I atom-doped MoTe2.

To gain more information on the structural changes caused by doping I atoms, the optimized geometric parameters of all doped systems are listed in Table ; bond lengths are denoted dTe–Mo and dI–Mo and bond angles are denoted ∠Te–Mo–Te and ∠Mo–I–Mo, as shown in Figure d,e. Meanwhile, the geometric parameters of pure monolayer MoTe2 are also listed for comparison. It can be seen that the lattice constant a of the optimized monolayer MoTe2 is 3.553 Å, which is in good agreement with the previous experimental[62] and theoretical calculation results,[63] indicating that the calculation method is reasonable and the calculation results are reliable. In addition, it is found that the geometric parameters in the I-doped MoTe2 systems are close to those in pure monolayer MoTe2, which is attributed to the I and Te atoms in the same period having similar atomic radii.[64,65]

Table 1

Calculated Lattice Constant a, Bond Lengths (dTe–Mo and dI–Mo), and Bond Angles (∠Te–Mo–Te and ∠Mo–I–Mo) of the Optimized MoTe2 Systems Doped with Different I Atom Concentrations, Respectively, Together with Those of Pure Monolayer MoTe2

configurations	lattice constant a (Å)	dTe–Mo (Å)	dI–Mo (Å)	∠Te–Mo–Te (deg)	∠Mo–I–Mo (deg)
pure MoTe2	3.553	2.736		80.991
1I-doped MoTe2	3.566	2.741	2.777	81.741	82.354
2I-doped MoTe2	3.542	2.734	2.758	81.481	79.880
3I-doped MoTe2	3.676	2.755	2.775	84.289	88.714

The transition from semiconducting to metallic behavior of I-doped MoTe2 systems can be associated with the impurity level introduced by doping. As shown in Figure a, the band structure of pure monolayer MoTe2 shows that the direct band gap at the highly symmetric G point is about 1.083 eV, which is consistent with the previous calculation results[62,63] and in good agreement with the experimental values.[35,66] In contrast to the semiconductor behavior in the pure monolayer MoTe2, I-doped MoTe2 systems appear to exhibit metallic behavior (Figure b–d). Compared with the PBE results, the HSE calculations significantly overestimate the band gap of pure MoTe2 by 50% (see Figure S1 and Table for details). In the simulations, the nonlocal feature of the HF exchange may be insignificant and HSE06 may not be an appropriate functional to use. The energy bands of I-doped MoTe2 systems are highly dense and show a significant shift to the lower energy area. Subsequently, impurity levels cross the Fermi level, making all doped systems exhibit metallic behavior, which is a typical characteristic of n-type doping.[67,68] In addition, with the increase in the I concentration, the bandwidth of I-doped MoTe2 systems decreases due to the increase in the impurity level close to the Fermi level. As a result, each donor impurity removes a state from the conduction band and establishes it as an impurity level of lower energy in the interband. With the increasing concentration of impurity I atoms, the whole conduction band shifts down to a greater extent, and finally, impurity levels cross the Fermi level. Electrons in these impurity levels can conduct in the same way as electrons in metals, which increases the probability of electron transition near the Fermi level.

Figure 2

Band structures of (a) pure and (b–d) different I atom concentration-doped monolayer MoTe2 calculated by the PBE method.

Table 2

Band Gap of Monolayer MoTe2 (eV)

structures	PBE	HSE	experimental[35,66]	other theoretical studies[62,63]
MoTe2	1.083	1.509	1.10	1.07	1.10

To further investigate the electronic properties, the total density of states (TDOS) and partial total density of states (PDOS) of pure and I-doped monolayer MoTe2 are calculated, as shown in Figure . The DOS shows that impurity states produce highly localized orbitals within the band gap of the pure MoTe2. In addition, the PDOS shows the electronic hybridization behavior among the Mo, Te, and I atoms. It can be found that the pure MoTe2 valence band is mainly composed of Te 5p and Mo 4d orbitals, while the conduction band is mainly composed of Mo 4d orbitals. The valence band shows the strong hybridization between Te 5p and Mo 4d orbitals with a bandwidth of approximately 6.0 eV. In addition, Te 5s states mainly emerge in the range of −12.5 to −8.4 eV, which is highly close to the band calculation results (Figure a(i),b). As shown in Figure a(ii)–(iv), I 5s states are observed in the energy range of −14.3 to −12.8 eV, which can be attributed to the strong localization of doped I atoms. Moreover, I 5s states are gradually enhanced with the increase in the I concentration. Figure b–e shows that Te 5s states have shifted slightly to the low-energy area after I doping. It should be noted that the number of Te atoms is decreased due to the substitution of I atoms, thus Te 5p states at the top of valence bands are slightly reduced. Upon I atom doping, both valence bands and conduction bands shift to the low-energy area; conduction bands are mainly composed of Mo 4d and Te 5p orbitals and valence bands are mainly dominated by Te 5p, Mo 4d, and I 5p orbitals, suggesting the possible interactions between the I and Te and Mo atoms. Specifically, 4d orbitals of Mo atoms in monolayer MoTe2 were strongly hybridized with 5p orbitals of I dopants due to the large overlap, and impurity levels introduced by hybridization cross the Fermi level, implying metallic behavior of all doped systems. In addition, the DOS of I-doped MoTe2 systems near the Fermi level is not zero, which may be due to the distribution of impurity levels induced by doping I atoms in the interband.

Figure 3

(a) TDOS and (b–e) PDOS of pure and different I atom concentration-doped monolayer MoTe2.

The charge density difference is calculated to investigate charge transfer between the I atoms and monolayer MoTe2. Figure displays the optimized stable configurations and their corresponding charge density differences for the doped systems. It can be seen that impurity atoms in the doped systems interact strongly with neighbor atoms and their charge distributions changed significantly. The yellow regions represent charge accumulation and the blue regions represent charge depletion. Interestingly, with the introduction of the I atom, the regions with strong bonding between I and Mo atoms are found in the I-doped MoTe2 systems,[69] which may indicate the covalent interaction between I and neighbor Mo atoms (Figure a–f). In addition, there is charge depletion around the I atoms and charge accumulation near the Mo atoms in the doped systems, that is, the I atoms lose electrons and Mo atoms gain electrons, which are consistent with the orbital hybridization between Mo and I atoms in the PDOS. Therefore, the change in the global electronic properties of I-doped MoTe2 systems is attributed to local changes in electron distribution.[70]

Figure 4

Top and side views of charge density differences of monolayer MoTe2 doped with (a, b) one, (c, d) two, and (e, f) three I atoms, respectively. Isosurfaces correspond to 0.003 e Å–3.

To better understand the optical transition of the doped systems, it is very imperative to investigate the imaginary part of dielectric function ε2(ω). ε2(ω) of pure and I-doped MoTe2 is shown in Figure a. Due to the hexagonal symmetry of monolayer MoTe2, ε2(ω) exhibits obvious anisotropy. For pure MoTe2, three distinct peaks are observed at 1.75, 2.82, and 3.53 eV. As a direct band-gap semiconductor, the spectrum of pure MoTe2 is generated by the electronic transition between energy levels, and each dielectric peak can be explained by its band structure and PDOS. Combined with the PDOS in Figure b, it is found that the main peak at 1.75 eV is mainly due to the intrinsic transition between Te 5p states in the highest valence band and Mo 4d states in the lowest conduction band, while main peaks at the corresponding position in I-doped MoTe2 systems are mainly derived from the electron transition among Mo 4d, I 5p, and Te 5p states. In addition, compared with pure MoTe2, the three peaks of all doped systems have significant red shifts, and the corresponding intensity decreases with the increase in the I doping concentration. The reason that the height of the main peak at 1.75 eV decreases with increasing I doping concentration is that the 5p orbitals of I increasingly occupy the dominant position in the energy band, which greatly reduces the probability of the electron hopping change process in the 4d state of Mo and the 5p state of Te. To sum up, these results imply that the bandwidth of I-doped MoTe2 systems is reduced, which is related to the local lattice distortion of a pure MoTe2 structure caused by the doping of impurity I atoms and the introduction of impurity levels in the doped systems. ε2(ω) of pure MoTe2 simulated using an HSE06 hybrid functional (Figure S2a) shows some new dielectric peaks, which are significantly deviated from the PBE simulation results. The HF exchange for simulating the optical properties of I-doped MoTe2 systems seems to be inappropriate.

Figure 5

Optical properties of pure and different I atom concentration-doped monolayer MoTe2 calculated by the PBE method: (a) imaginary part of the dielectric function ε2(ω) and (b) absorption spectra.

The simulations of absorption spectra suggest that a relatively high doping concentration can more effectively enhance the absorption ability of MoTe2 in the NIR-II window. As shown in Figure b, compared with pure monolayer MoTe2, the optical absorption of single I atom-doped MoTe2 in the NIR-II window is very weak, while two and three I atom-doped MoTe2 systems show considerable improvement in the NIR-II absorption. Specifically, the two I atom-doped MoTe2 shows an absorption peak at 1300 nm, while the three I atom-doped MoTe2 has an absorption peak at 1180 and an absorption enhancement at 1700 nm. The absorption amplitude of I-doped MoTe2 systems in the NIR-II window is significantly enhanced. According to the previous theoretical work,[71] the emission spectrum is the product of the absorption spectrum and quasi-Fermi distributions for electrons and holes. The absorption spectra are enhanced in the NIR-II region by iodine doping. The quasi-Fermi distribution, which is the production of occupied conduction states and unoccupied valence states can be improved by the semiconductor–conductor conversion: in I-doped MoTe2 systems, occupied conduction states and unoccupied valence states can be created much easier. Therefore, it can be inferred that the relatively high doping concentration can also improve emission in the NIR-II window. Among them, the absorption coefficient of the MoTe2 system doped with three I atoms increases significantly, which is due to the introduction of more impurity levels in the interband and thus effortless transition for electrons from impurity levels to conduction bands.

Conclusions

In summary, first-principles calculations have been employed to investigate the electronic and optical properties of the MoTe2 doped with different I concentrations. Compared with pure MoTe2, the bandwidth of I-doped MoTe2 systems decreases with increasing I doping. In addition, all doped systems show metallic properties because impurity levels induced by I doping cross the Fermi level. According to the analysis of optical properties, with the increase in the I doping concentration, the absorption amplitude of the doped systems in the NIR-II region is significantly improved and the quasi-Fermi distribution can also be improved, thus the emission spectra, which are the product of the absorption spectra and quasi-Fermi distributions for electrons and holes can be enhanced in the NIR-II window.

The theoretical simulation including the band structure and DOS may represent the actual electron structure due to the convergence between the simulated band gap and the experimental band gap of MoTe2. The first-principles algorithms do not reproduce the fluorescence characters at the theory level. Kasha’s rule[72] provides a bottom line of the link between absorption and emission but the details of fluorescence including the molecular oscillations in the emission process are still unclear. This work shows the potential of NIR-II emission materials, I-doped MoTe2. Experimental synthesis and measurements are needed to confirm the effectiveness of the emission in the future.