J Electroceram DOI 10.1007/s10832-016-0035-0
Controlling the electronic properties of Gd: MoS2 monolayer with perpendicular electric field Abdul Majid 1 & Arslan Ullah 1 & Tahir Iqbal 1 & Usman Ali Rana 2 & Salah Ud-Din Khan 2 & Masato Yoshiya 3
Received: 10 January 2016 / Accepted: 18 April 2016 # Springer Science+Business Media New York 2016
Abstract A systematic computational study to demonstrate electric field dependence of electronic properties of Gd doped MoS2monolayer is being reported. Density functional theory (DFT) based calculated were performed using ADF-BAND package to investigate the effects of applied electric field on pure and Gd doped monolayer of MoS2using supercell approach. A detailed analysis of electric field dependence of host and dopant related states in the monolayers was carried out and discussed to explore the possible implications in devices. The findings on the basis of calculated results indicate that band gap of the monolayer decrease with increase in value of applied electric field. A model indicating this behaviour is also reported. It was further revealed that the formation energy of the monolayers exhibits a consistent decrease with increase in electric field.
Keywords Monolayer . Band gap . Electric field . Energy applications
* Abdul Majid
[email protected] * Usman Ali Rana
[email protected]
1
Department of Physics, University of Gujrat, Gujrat, Pakistan
2
Sustainable Energy Technologies Center, College of Engineering, King Saud University, PO-Box 800, Riyadh 11421, Saudi Arabia
3
Department of Adaptive Machine Systems, Osaka University, Osaka, Japan
1 Introduction In recent decade, due to the progress in growth and characterization techniques in material science, demand for novel materials has been increased [1–3]. The search for new materials showing improved chemical and physical properties with potential of device applications has become a prime research interest. One of leading material emerged during this scientific struggle is known as two dimensional (2D) graphene which just after its discovery became the most studied material [4] and a lot of research has been reported on it [5–9]. Owing to its properties, it has found fascinating applications in electronics and optoelectronics like photocatalysis, solar cells, diodes and high speed devices [1–10]. It has been found that the pure graphene is a semiconductor with zero band gap [11], which put upsetting restrictions on its applications in several optoelectronic devices and forced the researchers to search some of its alternative materials. The outcome of resulting search appeared in the form of Transition Metal Dichalcogenides (MDs), which consist of 2D materials, with chemical formula MX2 (M: metal and X: chalcogens atom), having properties similar to that of graphene. The single layer of these TMDs consists of 3 layers of atoms where a metal atom is sandwiched between two sideways chalcogens atoms. These materials offer several exceptional properties and are potential materials for fabrication of hetro structures electronic devices and are potential materials for applications in different fields [12]. One member of MDs family is MoS2 whose monolayer is direct gap semiconductor and its band gap can be tuned by applying mechanical strain or perpendicular E-field [13]. The presence of native band gap and its electronic structure offer useful electrical properties in monolayer MoS2 based field effect transistors [14]. Initially it was thought that material properties can be tailored once it has been fabricated but in 1960 it was revealed
J Electroceram
that post-growth application of electric field may change crystal properties [15, 16]. The changes in electronic and magnetic properties on application of electric field were observed [17] which are of interest from view point of applications in spintronics. On the basis of recent findings it has been observed that MoS2 monolayer may be an efficient diluted magnetic semiconductor (DMS) for applications in memory elements and other spintronic devices [12]. Though few reports on electric field dependence of pure MoS2 are available [18, 19] but study of application of electric field on DMS MoS2 monolayer is yet to be reported. Like other DMS there has been a growing interest to explore magnetic properties in MoS2 layers [20–22]. Feng et al. doped Fe-X6 (X = S,C,N,O,F) clusters into MoS2 monolayers and reported half metallic as well as spin-gaplesssemiconducting behaviour in the material [20]. The substitutional doping of Fe, Mn and Co into MoS2 monolayer to obtain magnetic character of the dopants in neutral as well as charge states has been studied using first principles [21]. Mn doped MoS2 layers have been synthesized via vapour phase deposition to report modification in chemical, optical and magnetic properties of the material [22]. Rare earth (RE) doping into nano materials has been found a robust technique to introduce new functionalities in the materials [23]. The emergence of magnetic ordering has been reported into ZnO monolayers upon doping with Ce, Eu, Gd and Dy [24]. Unlike majority of renowned semiconducting hosts, negligible efforts have been reported to dope RE ions into MoS2 for exploiting the further potentials of the material. The substitutionally alloying of MoS2 and CeS2 monolayers has recently been reported [25]. First principles investigations revealed that Gd doped MoS2 is magnetic and carry ptype semiconducting properties [26]. Gd has been a straight forward choice as magnetic dopant since it offers maximum magnetic moment of 7/2 μB/atom due to its unique electronic configurations. Owing to possibility of tailoring the material properties by switching on the electric field, there has been great research interest in study of electric field dependent properties of the materials which are to be used in electronic/electrical devices. Motivated by these considerations and looking for unexplored properties of 2D MoS2 after doping with Gd and how it reacts in the presence of applied electric field, this study was devoted to investigate the modifications in electronic properties of Gd:MoS2 monolayers upon applications of electric field.
2 Computational details The results being reported were calculated using DFT procedure implemented within ADF-BAND package which operates under linear combination of atomic orbitals (LCAO) [27]. The input was set up by using graphical user
interface (GUI) of the code. BAND offers choice of dimensions to impose periodicity (1D for chain, 2D for slab and 3D for bulk) on structures for energy calculations under periodic boundary conditions. In order to study MoS2 monolayer, we chose a two dimensional slab extended in xy plane. The structure was taken as 36 atoms crystal supercell under periodic boundary conditions to perform calculations. Unlike most DFT codes, ADF uses Slater-type basis functions. The calculations were performed using all electron triple ζ Slater type orbitals plus double polarized basis set available in the code’s database. For structural relaxation and energy calculations, the Brillouin zone sampling was carried out using Monkhorst– Pack K-mesh 3 × 4 × 1 for all calculations. Exchange correlation functional of generalized gradient approximation functional under Perdew Burke Ernzerhof parameterization was employed. Hubbard U value of 8 eV was employed for dealing with strongly correlated system of the material [26, 28]. The hexagonal structure of pure MoS2 monolayer extended along xy-plane was chosen with lattice constants as: a = 3.16 Å and c = 12.30 Å [12]. The entire structures were relaxed and calculations were carried out using a total energy convergence criterion of 10−4 eV to obtain geometrically optimized structures. The entire calculations were fully self consistent. The integration accuracy parameter 4.5 was selected for achieving good numerical accuracy. The dopant Gd was substituted on Mo site in 36-atoms supercell. Frozen core approximation was used for which Mo:4s 2 4p 6 4d 5 5s 1 , S:3s23p4 and Gd:5s25p64f75d16s2 were taken as valance orbitals. Keeping open shell configuration of Gd into account, the calculations were run in unrestricted mode to consider spin polarization. The electric field was applied in steps of 0.5 V/Å perpendiculars to the basal plane of the layers until the transition from semiconductor to metal taken place.
3 Results and discussion 3.1 Electronic properties of Gd:MoS2 mono layer The calculated density of states (DOS) in the form of total TDOS and partial PDOS of pristine and Gd doped MoS2 monolayer are given in Fig. 1. Fermi level is positioned at 0 eV for sake of convenience. In case of pure MoS2 the calculated value of band gap is 1.74 eV which is in agreement with the literature [29]. It can be seen from the figure that by doping the MoS2 with Gd introduced new states in the band gap. The position of Fermi level in our ADF-BAND calculations agrees with findings of Zhang et al. who employed VASP code for calculating the DOS of Gd doped MoS2 [26]. Furthermore, their findings of reduction in bandgap of the material also corroborate with our results. In addition to their assignment of p-type doping, on
J Electroceram 5 0 2 -5 0 -2 0
DOS (States per eV)
2
Fig. 1 Total DOS of (top) Pure MoS2 and (bottom) Gd doped MoS2. Partial DOS of Gd-4f in red shaded and Gd-5d in blue shaded are also given. For sake of convenience, Fermi level is set at 0 eV
the basis of position of Fermi level we predict that Gd:MoS2 is a degenerate p-type semiconductor. Furthermore, doping has introduced spin polarization due to the fact that Gd 4f7 contains half-filled 4f shall with occupied spin up states and empty spin down states. It revealed that Gd:MoS2 is a DMS and may has maximum magnetic moment of 7/2 μB due to 7 unpaired spin-up electrons in the f-orbital. It points to possibility of using Gd doped MoS2 in spintronic devices like magneto electric charge trap memory and logic devices that consume very small power and nonvolatile memory cells [30, 31]. The comparison of DOS indicates the suppression of states after doping and reduction of band gap from 1.74 eV to 0.41 eV. The impurity band related to rare earth dopants often observed resonating with conduction band which produced band gap narrowing in semiconductors [32]. A decrease in bandgap or complete conversion to metallic nature upon doing with Gd has been observed using computational and experimental techniques [33]. Though, no experimental report is available on Gd doped MoS2 but similar studies have been reported for several other materials. The doping concentration dependent decrease in band gap of Gd doped SnO2–TiO2nanoparticles, prepared using ultrasonic and hydrothermal methods, from 5.3 to 2.0 eV has been reported [34]. A significant red shift in series of Gd-doped TiO2 nanoparticles has already been reported [35]. Like several other materials, double exchange interactions is cause of magnetic ordering in Gd:MoS2monolayer. The modification in electronic properties of MoS2 monolayers to exhibit half-metallic properties and spin gapless semiconducting behaviour upon incorporating iron containing clusters has been reported [20]. In order to further highlight the effect of doping, partial DOS related to host and dopant atoms are given in Fig. 2.It is obvious that d-states of Mo and Gd along with p-states of sulfur provides major contribution at the Fermi level.
0 -2 2 0 -2 2 0 2 -2 0 -2 0 2 0 -2 2 0 -2 2 0 -2
Total DOS Gd:MoS2 S-p
S-s
Mo-f
Mo-d
Mo-p
Mo-s
Gd-f
Gd-d
Gd-p Gd-s
-10
-5
0
5
10
Energy (eV) Fig. 2 Partial DOS related to host and dopant for Gd doped MoS2 monolayer
3.2 Application of perpendicular E-Field on Gd:MoS2 The fact that application of electric field modifies material properties [15, 16] has triggered rich research activities in field of electronic materials science. The results obtained on new materials are expected to play vital role in applications including memory elements, quantum computing and many low power volatile devices [36–38]. Electric field can affect the electronic and magnetic properties of material in a number of ways. The application of electric field splits the states which causes mixing of atomic orbitals thus lowering the CB and raising the VB to cause reduction of band gap in semiconductors [32]. In case of molecular materials, a shift in HOMO and LUMO has usually been observed by the application of electric field to cause reduction of band gap [39]. In order to investigate the effects of applied perpendicular electric field to tune the properties of Gd doped MoS2 monolayer, the calculations were carried out at different fields and the calculated DOS are plotted in Fig. 3. The results show a gradual decrease in bandgap with increase in value of applied electric field until at 6.5 V/Å when conduction band (CB) and valance band (VB) merge to cause semiconductor to metallic transition. A complete closing of band gap for pure MoS2 has been reported to happen at applied field of 1.50 V/Å [19].
J Electroceram 2
E=6.5 V/A
0
0.4
-2 5 E= 6.0
0
0.3
Band Gap (ev)
DOS (States per eV)
5
E = 5.0
0 -5 5
E = 4.0
0 -5 5 0 -5 5
0.1
E = 3.5
E = 2.5
0.0
0 -5 5 0 -5 5
E = 2.25
-1
1
2
3
4
5
6
7
E.Field (VA )
E = 1.5
Fig. 4 Band gap versus electric field for Gd:MoS2 monolayer
-5
-10
-5
0
Energy (eV)
5
10
Fig. 3 TDOS calculated for Gd:MoS2 monolayer at different values of perpendicular applied electric field
The analysis of DOS pointed out that most prominent states participating in the formation of CB and VB are p-states from S and dxy, dy2 and dy2-z2 states of Mo in MoS2. However, in case of Gd:MoS2, the CB shows major contribution from the d-states of Mo and Gd along with p-states from sulfur. By the application of applied electric field (E), the states split and cause mixing of atomic orbitals so there is shift towards the centre of band gap occurred for both conduction band and valance band that causes narrowing of band gap. By further increasing the value of BE^ Field more reduction in the band gap occurred, this continues till the value of 6.5 V/Å at which both bands merge with each other. This value is four times higher than previously reported value for pure MoS2 monolayer [19]. The variation of bandgap with applied electric field is plotted in Fig. 4. The curve fitting analysis indicates that the trend of variation follows exponential like behavior given in following model, E g ðEo Þ−Eg ðE ∞ Þ
E g ðE Þ ¼
e
E−E c dE
þ E g ðE ∞ Þ
Where, Eg E Eg(E0) Eg(E∞)
0
-1
0
Ec
0.2
Band gap energy Applied electric field Band gap energy at applied field of E = 0 Band gap energy at maximum applied electric field E=∞ Value of electric field at center of the curve, where slope is maximum
Our proposed material due to having tuneable electronic and magnetic properties can be exploited for applications in
thin film transistors, logical circuits and detectors. The findings of this study indicate that upon using suitable gatevoltage in devices one can switch on and off the polarization in the material. We report highest field of 6.5 V/Å to produce semiconductor to metallic transition in Gd:MoS2, which predicts its use for high field application.
4 Conclusions In summary, in order to explore band engineering prospects for MoS2 monolayers, we doped Gd into the layers and applied perpendicular electric field of different values by using first principles methods. We observed spin polarization and a notable decrease in band gap of the material after doping. The application of electric field to Gd:MoS2 caused a consistent reduction of bandgap until the semiconducting material turned conductor at 6.5 V/Å.
Acknowledgments Abdul Majid, international research fellow of Japan Society for the promotion of science (JSPS), acknowledges financial support from JSPS. U. A. Rana would like to extend his sincere appreciation to the Deanship of Scientific Research at the King Saud University for its funding of this research through the Prolific Research Group, Project No. PRG-1436-18.
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