Surface Charge Transfer Doping via Transition Metal Oxides for

Oct 31, 2016 - *E-mail: [email protected]. ... SCTD offers the possibility to create a variety of electronic and optoelectronic devices from the II–...
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Surface Charge Transfer Doping via Transition Metal Oxides for Efficient p‑Type Doping of II−VI Nanostructures Feifei Xia,†,⊥ Zhibin Shao,†,⊥ Yuanyuan He,†,‡ Rongbin Wang,†,§ Xiaofeng Wu,† Tianhao Jiang,† Steffen Duhm,† Jianwei Zhao,‡ Shuit-Tong Lee,† and Jiansheng Jie*,† †

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, Jiangsu, People’s Republic of China ‡ Materials and Textile Engineering College, Jiaxing University, Jiaxing 314001, Zhejiang People’s Republic of China § Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany S Supporting Information *

ABSTRACT: Wide band gap II−VI nanostructures are important building blocks for new-generation electronic and optoelectronic devices. However, the difficulty of realizing p-type conductivity in these materials via conventional doping methods has severely handicapped the fabrication of p−n homojunctions and complementary circuits, which are the fundamental components for highperformance devices. Herein, by using first-principles density functional theory calculations, we demonstrated a simple yet efficient way to achieve controlled p-type doping on II−VI nanostructures via surface charge transfer doping (SCTD) using high work function transition metal oxides such as MoO3, WO3, CrO3, and V2O5 as dopants. Our calculations revealed that these oxides were capable of drawing electrons from II−VI nanostructures, leading to accumulation of positive charges (holes injection) in the II−VI nanostructures. As a result, Fermi levels of the II−VI nanostructures were shifted toward the valence band regions after surface modifications, along with the large enhancement of work functions. In situ ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy characterizations verified the significant interfacial charge transfer between II−VI nanostructures and surface dopants. Both theoretical calculations and electrical transfer measurements on the II−VI nanostructure-based field-effect transistors clearly showed the p-type conductivity of the nanostructures after surface modifications. Strikingly, II−VI nanowires could undergo semiconductorto-metal transition by further increasing the SCTD level. SCTD offers the possibility to create a variety of electronic and optoelectronic devices from the II−VI nanostructures via realization of complementary doping. KEYWORDS: surface charge transfer doping, SCTD, p-type doping, II−VI nanostructures, transition metal oxides, field-effect transistors acceptor levels,12 complementary doping on II−VI nanostructures remains difficult and challenging.13 The p−n homojunctions are essential components for high-performance optoelectronic devices because of matched band structures and minimal junction interface defects.14 However, the lack of complementary doping renders it impractical to construct highquality p−n homojunctions from II−VI nanostructures. Additionally, while high carrier mobilities of II−VI nanostructures

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ide band gap II−VI nanostructures have attracted considerable attention in the past decade due to their superior electronic and optoelectronic properties.1−8 They have found important applications in diverse fields, including field-effect transistors (FETs),1,2 light-emitting diodes (LEDs),3,4 photodetectors,5,6 biosensors,7,8 and so on. Despite the great progress, the lack of understanding and controllability on the electrical properties of II−VI nanostructures severely impedes their practical applications. Most of the II−VI nanostructures exhibit unipolar electrical conductivity, that is, n-type conductivity for ZnS, ZnSe, CdS, and CdSe and p-type conductivity for ZnTe and CdTe.9 Due to a strong selfcompensation effect,10 low solubility of dopants,11 and deep © 2016 American Chemical Society

Received: August 31, 2016 Accepted: October 31, 2016 Published: October 31, 2016 10283

DOI: 10.1021/acsnano.6b05884 ACS Nano 2016, 10, 10283−10293

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ACS Nano offer great device potential,15,16 the lack of complementary doping has severely hampered the construction of integrated circuits consisting of both n- and p-type FETs.1,17,18 Due to the high surface-to-volume ratio of semiconductor nanostructures, surface effect plays a key role in determining their optical,19 mechanical,20 catalytic,21 and optoelectronic properties.22 Recent investigations demonstrated the feasibility to manipulate the electrical properties of semiconductor nanostructures through surface charge transfer doping (SCTD).23,24 In this approach, through controlling Fermi level (EF) misalignment of surface dopants with respect to underlying semiconductor nanostructures, electrons can be extracted from (or injected into) the nanostructures,25 forming an electrondeficient (or electron-rich) surface layer. Carrier concentration and even conduction type of the semiconductor nanostructures can be readily tuned by varying the types as well as densities of surface dopants, leading to effective n- and p-type doping on the nanostructures. In comparison with conventional substitutional doping methods, the SCTD approach is not restricted by the aforementioned unfavorable factors in conventional doping methods, and more importantly, it is nondestructive and does not induce any bulk defects into the semiconductor lattice, thus retaining the high performance of nanostructures by reducing carrier scattering in the bulk. The p-type SCTD mechanism was first proposed to explain the surface p-type conductivity in the hydrogen-terminated diamond, in which an adsorbed water layer acts as an electron acceptor.26 Subsequent studies were focused on carbon nanotubes27−29 and two-dimensional (2D) materials such as graphene 30−34 and transition metal dichalcogenides (TMDs)35−40 as they possess atomic thickness and are particularly susceptible to surface dopants. SCTD shows the large capability to rationally tune the electronic, optical, and optoelectronic properties of various 2D nanostructures. For instance, deposition of high work function MoO3 thin film on epitaxial graphene (EG) induced significant electron transfer from graphene to MoO3.31 As a result, a hole accumulation layer with an areal hole density of about 1.0 × 1013 cm−2 was achieved for the MoO3-modified EG layer. The substoichiometric MoOx (x < 3) could also serve as a surface dopant/ contact layer for hole injection into MoS2 and WSe2,35,36 making it possible to construct high-performance complementary circuits based on TMDs. In addition, the photoluminescence (PL) properties of MoS2 strongly depended on the surface status. Accumulation and depletion of electrons in MoS2 by SCTD resulted in the shift of peak position as well as the change in emission intensity.41−43 SCTD on the 2D materials of graphene and MoS2 revealed that the influence depth of

surface dopants in the perpendicular direction was ca. 1.5 nm, corresponding to 2−4 layers of the 2D materials.44−46 Although the SCTD scheme has been demonstrated to be effective for 2D materials, little work has been implemented for other lowdimensional nanostructures such as semiconductor nanowires (NWs)47−52 and nanoribbons (NRs),53 presumably due to the relatively large diameter/thickness of these nanostructures. Herein, based on first-principles density functional theory (DFT) calculations, we propose a strategy to achieve efficient p-type doping on II−VI nanostructures via SCTD by taking advantage of a series of high work function transition metal oxides (MoO3, WO3, CrO3, and V2O5) as surface dopants. Our calculations reveal that the oxide molecules can act as strong acceptors to draw electrons from II−VI nanostructures, leading to effective injection of holes into II−VI nanostructures. The downshift of Fermi levels toward valence bands, along with the increase of work functions, contributes to the p-type conductivity of the II−VI nanostructures. Characterizations using in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) confirm the remarkable interfacial charge transfer between II−VI nanostructures and surface dopants. Interestingly, the effect of SCTD is strong enough to fully convert the II−VI nanostructures to p-type conductivity and further to metallic conductivity, as revealed by electrical transfer measurements. The realization of reliable p-type doping by SCTD should be important for the development of a host of high-performance electronic and optoelectronic devices based on II−VI nanostructures.

RESULTS AND DISCUSSION To understand the interaction between transition metal oxide molecules (MoO3, WO3, CrO3, and V2O5) and (100) surfaces of ZnS, ZnSe, CdS, and CdSe, the energetically most favorable adsorption configurations are studied, as shown in Figure 1 and Figure S1. Taking MoO3 as an example, MoO3 molecules prefer to be stabilized on the basal (100) planes of ZnS, ZnSe, CdS, and CdSe with its O atoms pointing to the metal atoms in II− VI semiconductors (Figure 1). The optimized vertical distance between the MoO3 molecules and the (100) surfaces of II−VI semiconductors is on average ca. 2.05 Å. The computed adsorption energy (ΔE) is ca. −2.00 eV for one MoO3 molecule adsorption on the surfaces of II−VI semiconductors, revealing a strong interaction between the MoO3 molecules and the (100) surfaces (Table 1). In analogy to MoO3 molecule, calculations on WO3 and CrO3 molecules also reveal similar adsorption configurations with different adsorption energies of ca. −2.5 and −1.2 eV, respectively (Table S1). However, for the adsorption of

Figure 1. Top and side views of the energetically favorable configurations of MoO3-modified II−VI semiconductors. 10284

DOI: 10.1021/acsnano.6b05884 ACS Nano 2016, 10, 10283−10293

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ACS Nano Table 1. Adsorption Energy (ΔE), Charge Transfer (Δq),a and Change of Work Function (ΔΦ)b of the MoO3Modified II−VI Semiconductors at the GGA/PBE Level geometries MoO3 MoO3 MoO3 MoO3

on on on on

ZnS(100) ZnSe(100) CdS(100) CdSe(100)

ΔE/eV

Δq/e

ΔΦ/eV

−2.38 −2.16 −2.24 −1.95

−0.20 −0.21 −0.29 −0.27

0.24 0.24 0.27 0.22

offer an effective approach to modulate the carrier concentrations in II−VI semiconductors. In addition, the changes in carrier concentrations will be reflected by the variations of work functions in semiconductors. Therefore, electrostatic potential calculations are further performed to probe changes of work functions (ΔΦ) for II−VI semiconductors before and after transition metal oxide molecule modifications. As shown in Table 1, adsorption of one MoO3 molecule on the (100) surfaces of ZnS, ZnSe, CdS, and CdSe in a 2 × 2 × 1 slab yields an obvious increase of work function by 0.24, 0.24, 0.27, and 0.22 eV, respectively, due to the hole injections from surface molecules into the II−VI semiconductors. To investigate the impact of surface coverage on the charge transfer, Mulliken charge analysis and electrostatic potential calculations of CdS with the different number of MoO3 molecules on the surface are performed, as shown in Figure 2. It is observed that both the positive charge of CdS and negative charge of MoO3 molecules increase as the number of MoO3 molecules increases (Figure 2a). Accordingly, with the increase of MoO3 coverage, the work function of CdS increases remarkably (Figure 2b). These results indicate that the higher surface density of MoO3 molecules, the more charge transfer between CdS and MoO3 molecules occurs. ΔE, Δq, and ΔΦ for two MoO3 molecules on the (100) surfaces of II−VI semiconductors are nearly twice those in one MoO3-moleculeadsorbed systems (Figure 2 and Table S1). However, it is noteworthy that the charge transfer and work function change of MoO3-modified II−VI semiconductors are not proportional to the MoO3 molecule number; they tend to saturate as the number of MoO3 molecules is further increased. Such a phenomenon is probably attributed to the saturated coverage of MoO3 molecules on the simulated surface of 2 × 2 II−VI semiconductor bulks. In practical experiments, the oxide layer with a substoichiometric component is usually obtained due to the loss of O atoms during the film deposition process such as vacuum evaporation.56 To gain an insight into this issue, we further calculated the charge transfer of Mo2O5 (corresponding to the loss of 0.5 O for each MoO3 molecule) and MoO2 (corresponding to the loss of 1 O for each MoO3 molecule) molecules with II−VI semiconductors (Figure S4). The calculated

a A negative Δq indicates that MoO3 molecules act as acceptors and electrons transfer from the (100) surface of II−VI semiconductors to MoO3 molecules. bΔΦ is defined as ΦMoO3/(100) − Φ(100), where ΦMoO3/(100) and Φ(100) are the work functions of MoO3-modified and intrinsic (100) surfaces of II−VI semiconductors, respectively.

the V2O5 molecule, the energy preferable geometry is a little bit different from those of MoO3-, WO3-, and CrO3-modified structures. Besides the O atom pointing to the metal atoms of II−VI semiconductors, a V atom in the V2O5 also points to the S or Se atoms of II−VI semiconductors (Figure S1). In addition, a larger adsorption energy of 2.6−3.2 eV is revealed for the V2O5 molecule because of the larger atom number in the molecule. Charge transfer between oxide molecules and (100) surfaces of II−VI semiconductors is studied by calculating the atomic charge on the intrinsic and adsorbed systems using a Mulliken charge analysis of bond population.54,55 From the tabulated atomic charges shown in Table 1, in the MoO3-modified II−VI semiconductor systems, the charge transfers (Δq) from (100) surfaces of ZnS and ZnSe to MoO3 molecules amount to −0.20 and −0.21 e, respectively, while those of CdS and CdSe are −0.29 and −0.27 e, respectively, revealing that the MoO3 molecule can act as strong acceptors on the semiconductor surfaces. In comparison with the isolated MoO3 molecules, the total Mulliken charges of the adsorbed MoO3 molecules on the II−VI semiconductors are increased by −0.20 e, resulting from the charge transfer from II−VI semiconductors to MoO3 molecules. Eventually, the adsorption of oxide molecules leads to positively charged (100) surfaces of II−VI semiconductors and negatively charged MoO3 molecules. The strong charge transfer suggests that surface modifications by MoO3 molecules can

Figure 2. Plots of (a) Mulliken charge analysis on MoO3 molecules and the CdS substrate, (b) work function (Φ) and change of work function (ΔΦ) as a function of MoO3 molecule number, (c) charge transfer (Δq), and (d) change of work function (ΔΦ) for transition metal oxide modified II−VI semiconductors at the GGA/PBE level. 10285

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Figure 3. Total DOS (red lines) of the transition metal oxide modified and intrinsic II−VI semiconductors. (a−d) PDOS of II−VI semiconductors (blue lines) and transition metal oxides (green lines) for MoO3-, WO3-, CrO3-, and V2O5-modified ZnS, ZnSe, CdS, and CdSe at the GGA/PBE level, respectively. The dashed line indicates the positions of EF. Orange arrows indicate the acceptor states introduced from the adsorbed transition metal oxides.

below the conduction bands, indicating the n-type conductivities of the intrinsic semiconductors. In contrast, the Fermi levels shift downward to the top of valence bands after oxide molecule adsorption, suggesting that the II−VI semiconductors have been tuned into p-type conductivities due to the SCTD effect. Moreover, new acceptor states are observed above Fermi levels, which are mainly generated by the adsorbed oxide molecules according to the PDOS spectra. Band structures of the II−VI semiconductors also reveal the generation of new acceptor levels within the band gap after the adsorption of transition metal oxides, which are indicated by the introduced yellow lines in Figure S2. Collectively, the above results demonstrate that the II−VI semiconductors have been effectively p-type-doped via SCTD upon the adsorption of MoO3, WO3, CrO3, and V2O5 molecules. To visualize the charge transfer between transition oxide molecules and II−VI semiconductors, we further calculated the difference charge densities (Δρ) of MoO3-modified ZnS, ZnSe, CdS, and CdSe, as shown in Figure 4. The Δρ is defined as Δρ = ρMoO3/(100) − ρ(100) − ρMoO3, where ρMoO3/(100), ρ(100), and ρMoO3 are the charge densities of the MoO3-modified systems, the intrinsic II−VI semiconductors, and the isolated MoO3 molecule, respectively. The regions of electron accumulation and depletion are displayed in red and blue colors, respectively. It is worth noting that there is a strong electron accumulation around O atoms of the MoO3 molecules, while the electron depletion appears around the (100) surface of ZnS, ZnSe, CdS, and CdSe. As a result, the charges on the adsorbed MoO3 molecules and II−VI semiconductors are negative and positive, respectively. This phenomenon indicates that the adsorbed MoO3 molecules gain electrons, whereas the II−VI semiconductors lose electrons. In another word, the holes have been injected into the II−VI semiconductors upon surface adsorption. On the basis of Mulliken population analysis, the charges on the MoO3-modified II−IV semiconductors are ca. 0.20 e, which indicates that a fractional positive charge (∼0.20 e) is injected into the II−IV semiconductors’ supercell. All these

results indicate that both Mo2O5 and MoO2 can act as acceptors to draw electrons from II−VI semiconductors, although the extent of charge transfer is slightly lower than that of the stoichiometric MoO3 molecule due to the loss of O atoms. Therefore, to simplify the calculations, only metal oxides with stoichiometric structures were studied in the following discussions. Besides the MoO3 molecule, charge transfer between other high work function transition oxide molecules (i.e., WO3, CrO3, V2O5) and II−VI semiconductors is also investigated. As shown in Figure 2 and Table S1, they all exhibit strong capabilities to draw electrons from II−VI semiconductors. As a result, the work functions of II−VI semiconductors increase remarkably upon surface molecule modifications. It is noteworthy that MoO3, WO3, and CrO3 exhibit similar capability to perform SCTD on II−VI semiconductors with CrO3 slightly stronger than the other two oxides. Nevertheless, the effect of V2O5 is much stronger; the charge transfer (work function tuning) for V2O5 is about twice (triple) that of the other oxides. This is due to the different adsorption structure of V2O5 on semiconductor surfaces. Compared to the molecular structures of MoO3, WO3, and CrO3, the V2O5 molecule has two additional O atoms and one more metal atom, which means that it can draw more electrons from II−VI semiconductors, thereby resulting in an enhanced SCTD effect. On the other hand, Figure 2c shows that the adsorption of transition metal oxides on CdS gives rise to the largest SCTD effect among the four II−VI semiconductors. Due to the significant charge transfer, the electronic properties of II−VI semiconductors can be dramatically changed upon the adsorption of transition oxide molecules. To gain insight into this point, we computed both the density of states (DOS) of transition metal oxide modified II−VI semiconductor systems and the projected density of states (PDOS) for II−VI semiconductors and transition metal oxides in these adsorption systems, as shown in Figure 3. For intrinsic II−VI semiconductors (before adsorption), the Fermi levels are located 10286

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Figure 4. Difference charge densities of MoO3-modified (a) ZnS, (b) ZnSe, (c) CdS, and (d) CdSe. Electron loss is displayed in blue, and electron enrichment is in red.

Figure 5. (a) XPS core level spectra (Cd 3d and Zn 2p) and (b) low kinetic energy region of UPS spectra for intrinsic and MoO3-modified CdS and ZnSe NWs. (c) Schematic energy level alignments of CdS, ZnSe, and MoO3 according to literature values.57,61,62

results verify that there is an obvious charge transfer between II−VI semiconductors and MoO3 molecules. Similar results were also found for WO3-, CrO3-, and V2O5-modified II−VI nanostructures (Figure S3). In this work, high work function transition metal oxides (MoO3, WO3, CrO3, and V2O5) are chosen as p-type surface dopants to achieve efficient p-type doping on II−VI semiconductors. The archetypal d0 transition metal oxides have been commonly used as hole injection layers in organic optoelectronics such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs),57,58 as they can be readily deposited in vacuum at a relatively low temperature (