Surface Charge Transfer Doping of Monolayer Phosphorene via

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Surface Charge Transfer Doping of Monolayer Phosphorene via Molecular Adsorption Yuanyuan He, Feifei Xia, Zhibin Shao, Jianwei Zhao, and Jiansheng Jie J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01920 • Publication Date (Web): 06 Nov 2015 Downloaded from http://pubs.acs.org on November 12, 2015

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The Journal of Physical Chemistry Letters

计算机 Surface Charge Transfer Doping of Monolayer Phosphorene via Molecular Adsorption Yuanyuan He†, Feifei Xia†, Zhibin Shao†, Jianwei Zhao‡, Jiansheng Jie†,*

†Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, Jiangsu, P. R. China. ‡Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 21008, Jiangsu, P. R. China.

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ABSTRACT. Monolayer phosphorene has attracted much attention owing to its extraordinary electronic, optical, and structural properties. Rationally tuning the electrical transport characteristics of monolayer phosphorene is essential to its applications in electronic and optoelectronic devices. Herein, we study the electronic transport behaviors of monolayer phosphorene with surface charge transfer doping of electrophilic molecules, including 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), NO2, and MoO3, using density functional theory combined with the nonequilibrium

Green’s

function formalism.

F4TCNQ

shows

the

optimal

performance on enhancing the p-type conductance of monolayer phosphorene. Static electronic properties indicate that the enhancement is originated from the charge transfer between adsorbed molecule and phosphorene layer. Dynamic transport behaviors demonstrate additional channels for holes transport in host monolayer phosphorene were generated upon the adsorption of molecule. Our work unveils the great potential of surface charge transfer doping in tuning the electronic properties of monolayer phosphorene and is of significance to its application in high-performance devices.

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TOC GRAPHICS

BP F4TCNQ/BP NO 2/BP

V DS = 2.0 V 36

MoO 3/BP

IDS (µA)

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24 12 0 -1.0

-0.5

0.0

0.5

VG (V)

3


1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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In recent years, graphene, as a representative of two-dimensional (2D) materials consisting of single-atomic layer, has attracted tremendous attention in the field of nanoelectronics, owing to its distinct structural, electronic, and optical properties.1-3 However, the absence of a band gap has limited the progress of graphene-based technologies.4 Scientists have explored the alternative 2D materials for a long period, in order to overcome the drawbacks of graphene. But among the other materials, silicene5 and germanene6 are not stable in the air and thus make them against the application in practical electronic devices. Although transition metal dichalcogenides (TMDs) are a class of direct-band gap semiconductors that are emerging as strong candidates in next-generation nanoelectronic devices,7 but monolayer TMDs FET may not be ideal for high-performance applications due to their heavy electron effective

masses

and

low

mobilities.8

Black

phosphorus

is

the

known

thermodynamically stable form of phosphorus. Similar to graphite, black phosphorus is made of vertically stacked layers of phosphorene, held together by weak van der Waals forces.9 Unlike graphene and other group IV elemental sheets, phosphorene possesses a unique, vertically skewed/wrinkled hexagonal structure, where each phosphorus (P) atom is covalently bonded to three other P atoms, resembling a steep washboard-like structure.10 Few-layer phosphorene has a direct bandgap, tunable from 1.51 eV for a monolayer to 0.5 eV for a five-layer sample.11 Its mobility is rather highly hole-dominated (∼104 cm2/Vs) and anisotropic. Although black phosphorus has been found as early as in 1910s,12 the applications of phosphorene in 4


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nanoelectronics and optoelectroincs are realized till 2014.13 Through mechanically exfoliating phosphorene from bulk of black phosphorus, Zhang et al. fabricated FETs based on few-layer phosphorene crystals with thickness down to a few nanometers, revealing the high device performance at room temperature.14 Despite the great progress of the experimental investigations are mainly based on few-layer phosphorene, monolayer phosphorene-based devices have not been achieved by experimental means hitherto as it is difficult to obtain monolayer phosphorene with large scale during the exfoliation process.15 Considering the strong layer-dependent properties of phosphorene, it is much desirable to study the performance of monolayer phosphorene-based devices. To date, some studies based on density functional theory (DFT) have reported the static electronic properties of monolayer phosphorene.16-19 The band gap of monolayer phosphorene experiences a direct-indirect-direct transition when axial strain is applied. Such static electronic properties demonstrate the great potentials for monolayer phosphorene to be applied in nanoelectronics. However, the dynamic electronic transport behaviors of monolayer phosphorene, directly reflecting the device performance, have not been clarified yet. It is well known that monolayer phosphorene is a p-type semiconductor with hole mobility superior to other layered materials such as MoS2 and WSe2.20,21 The performance of monolayer phosphorene based nanodevices relies on the capability of rationally tuning its electronic and transport properties.22,23 The conventional doping 5



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method takes effect by incorporating dopants into the host semiconductor lattices, but the damage in lattice structure as well as limited dopant solubility restricts the application of this method in 2D phosphorene. Alternatively, surface charge transfer doping method, which is based on surface charge transfer/injection mechanism, shows feasible as a nondestructive method to tune the electronic properties of 2D semiconductor materials, due to their high surface-to-volume ratio.24 Previous investigations revealed that the conductance of 2D semiconductor nanomaterials could be tuned upon the adsorption of electrophilic molecules, including inorganic molecules25, gas molecule26, organic molecules27 and so on28. For instance, Xu et al. have presented both n- and p-type doped exfoliated graphene sheets by virtue of adsorbing organic molecules, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and vanadyl phthalocyanine (VOPc).29 By DFT calculations, they also found that the charge transfer between F4TCNQ and graphene can open the band gap of graphene.30 We also demonstrated change of the conduction type of CdS nanoribbons from n-type to p-type via surface charge transfer doping.31,32 Nevertheless, surface charge transfer doping in monolayer phosphorene has been rarely studied thus far. Kulish et al. theoretically presented that phosphorene forms strong bonds with all studied adatoms while still preserving its structural integrity.33 Du et al. have found that surface decoration of MoO3 on phosphorene led to a largely enhanced photodetection behavior.34 Their results offer a possible route to tune the

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electronic properties of monolayer phosphorene by surface charge transfer doping for high-performance device applications. Herein, by combining density functional theory (DFT) and the nonequilibrium Green’s function formalism (NEGF), we carry out a systematic study on the static electronic as well as dynamic transport properties of monolayer phosphorene systems with the adsorption of various electrophilic molecules, including organic molecule F4TCNQ, inorganic molecule MoO3, and gas molecule NO2. It is found that F4TCNQ exhibit most significant p-type doping effect due to the strong charge transfer between the molecule and phosphorene layer. The transmission study demonstrates that the adsorbed molecule offers additional pathways for hole transport in devices. The enhanced p-type transport of monolayer phosphorene upon molecule adsorption is further clarified by the electrical characteristics of simulated devices. The rational tuning of electronic properties by surface charge transfer method paves the way toward high-performance nanodevices based on monolayer phosphorene. The calculations followed two steps. First, the DFT methodology was used to study the structural and electronic properties of monolayer phosphorene before and after adsorbing electrophilic molecules. Three kinds of electrophilic molecules, including F4TCNQ, NO2, and MoO3, were selected in this study since they offer excellent p-type doping effects in previous reports.35-37 The geometry optimization and charge transfer were performed by the ultrasoft pseudopotentials plane-wave method implemented in the Cambridge Sequential Total Energy Package (CASTEP) program 7



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from Accelrys, Inc.38,39 The generalized gradient approximation (GGA)40 with the Perdew–Burke–Ernzerhof (PBE) functional was adopted to describe the correction of the electronic exchange and correlation effects. In Figure S8, we have optimized geometries of F4TCNQ/BP system before and after considering van der Waals corrections, it shows that inclusion of van der Waals corrections has negligible effect on the geometries of molecule/BP system and the relative strength of their adsorption energies to each other. The structural model for monolayer phosphorene was periodic in the x, z directions and separated by at least 15 Å along the y direction to avoid the interactions between adjacent layers. The convergence criteria for geometric optimization and energy calculation were set as 2.0 × 10−5 eV/atom for the tolerance of energy, 0.02 eV/Å for the maximum force, 0.005 Å for the maximum ionic displacement, and 2.0×10−6 eV/atom for the self-consistent field (SCF), respectively. The Brillouin zone integration was sampled by a 10 × 8 × 1 k-grid mesh for a 3 × 3 supercell. An energy cutoff of 400 eV was chosen for the plane wave basis. Spin polarization was included in the calculations of the adsorption of NO2 because it is paramagnetic. Subsequently, the electronic transport properties were studied with the Atomistix Toolkit (ATK) software41, which was based on first-principles functional theory (DFT) combined with the nonequilibrium Green’s function (NEGF) formalism. The studied structures had been treated as two-probe systems with the central scattering region sandwiched between semi-infinite source (left) and drain (right) electrode regions, in 8


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which electrophilic molecules are adsorbed on its top surface. A 2 × 4 supercell without molecular adsorption was used for each of the left and right electrodes, while the center scattering was considered in a 4 × 9 supercell with molecular adsorption, which was long enough to damp out charge oscillations at the end layer that resulted from the charge dipoles between P atom and adsorbed molecules under electric field.42 To describe the interactions of the valence and core electrons, we used norm-conserving pseudo-potentials, as proposed by Troullier-Martins.43 The valence electronic orbitals of systems were described using a single-ζ basis set plus a polarization function (SZP). A sample of 4 × 1 × 100 k-points chosen to the Monkhorst-Pack method was used to describe the Brillouin zone and the mesh cutoff was chosen as 200 Ry to achieve a reasonable balance between the calculation efficiency and accuracy. The geometry including the P atoms and adsorbed molecule in the central region was fully optimized by minimizing the atomic forces on those atoms to be smaller than 0.10 eV/Å. Due to the symmetry of the junction, the bias was scanned only in one direction from 0.0 to 2.0 V.

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Figure 1. (a) Charge transfers between monolayer phosphorenes and the adsorbed F4TCNQ, MoO3, and NO2 molecules. The isosurface value for all of the cases is e/Å3. The red isosurface indicates an electron gain, whereas the blue one represents an electron loss. (b) Potential gradients of monolayer phosphorene systems with different adsorbed molecules at 1.0 V and 2.0 V, respectively.

To understand the relationship between the adsorption of molecules and the static electronic properties of monolayer phosphorene, we first investigated the geometries, electronic structures, and energetics of monolayer phosphorene with different adsorbed molecules based on first principle method. As shown in Figure 1a, a 3 × 3 supercell was used to model phosphorene layer, which has lattice dimensions of 9.898 Å × 13.854 Å, in good agreement with previous reports,44 and the z axis is along armchair direction. For each adsorption case, a molecule is placed near the phosphorene layer, and the whole system is full relaxed. All of the relaxed structures 10


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with different adsorbed molecules are presented in Figure 1a. Because of the highly symmetrical geometry, the most stable configuration for F4TCNQ is the top site, with its long axis along the z-direction of unit cell. The equilibrium distance between F4TCNQ and phosphorene is 2.29 Å. Meanwhile, the adsorbed F4TCNQ molecule is severely twisted from flat plane due to weak interaction of physisorption. For the adsorption of NO2 and MoO3 molecules, they are both located at the center of puckered honeycomb. The Mo atom in MoO3 molecule can form a direct bond of 2.50 Å with the P atom in monolayer phosphorene. However, NO2 does not bond with the P atom due to the lack of dangling bonds and, consequently, stays above the phosphorene layer with an adsorption distance of 1.79 Å. Previous studies had shown that molecular adsorption on 2D semiconductor may affect its electronic properties, and further realize its doping effect.45 In order to examine how these adsorbed molecules affect the charge transfer between the adsorbed species and monolayer phosphorene, we calculated the charge transfer between adsorbed molecules and phosphorene layer under the electric field along the z axis, which is defined as

∆ρ = / − −

(1)

where / , , and represent the charge densities of the molecule-adsorbed system, the isolated molecule, and phosphorene without molecular adsorption, respectively. The charge transfers of F4TCNQ-adsorbed phosphorene (F4TCNQ/BP), MoO3-adsorbed phosphorene (MoO3/BP), and NO2-adsorbed 11



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phosphorene (NO2/BP) systems at the bias of 0 V are shown in Figure 1a. The regions of electron accumulation and depletion are displayed in red and blue colors, respectively. It can be noted that there is a strong electron accumulation around N atoms in F4TCNQ molecule, whereas electron depletion appears around the P atoms in phosphorene layer. This phenomenon suggests that the adsorbed F4TCNQ molecule gains electrons, but the monolayer phosphorene loses electrons. Comparing the charge transfers in monolayer phosphorene systems with different adsorbed molecules, we find that F4TCNQ/BP system has the largest amount of charge transfer of all. The binding energy (Ea) of monolayer phosphorene system with molecular adsorption is defined as:

= − −

(2)

where / , , and represent the total energies of the molecule-adsorbed system, the isolated molecule, and monolayer phosphorene without molecular adsorption, respectively. F4TCNQ molecule has the largest binding energy of 4.04 eV/unit cell to monolayer phosphorene compared to the other electrophilic molecules studied in the present work. The largest amount of charge transfer in F4TCNQ/BP system is attributed to that the adsorption of F4TCNQ on monolayer phosphorene is the most stable. The charge changes of monolayer phosphorene devices before and after adsorbing molecules at different applied biases are shown in Table 1. Total value of the charges distributed on all P atoms in monolayer phosphorene is almost zero, whether or not 12


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the bias is applied. Without the application of bias, the adsorbed F4TCNQ is significantly negatively charged (-0.911 e) and monolayer phosphorene is remarkably positively charged (0.800 e), which indicates that a large number of electrons are withdrawn from monolayer phosphorene to the absorbed F4TCNQ molecule and as a result, a lot of holes are injected into phosphorene layer. But the absorbed MoO3 and NO2 molecules are less negatively charged (-0.268 e and -0.251 e, respectively) and monolayer phosphorenes are less positively charged (0.263 e and 0.176 e). It shows that F4TCNQ has the strongest electron-withdrawing ability compared to NO2 and MoO3 molecules. When the applied bias rises from 1.0 V to 2.0 V, the charge of F4TCNQ becomes more negative (from -0.933 e to -0.940 e). However, MoO3 and NO2 molecules are less negatively charged (from -0.265 e to -0.259 e on MoO3, from -0.256 e to -0.248 on NO2). It is believed that F4TCNQ molecule is an excellent p-type dopant to inject holes into monolayer phosphorene and its electron withdrawing ability can be improved under a higher bias.

Table 1. Charge changes (e) of monolayer phosphorene devices before and after adsorbing molecules. Charge Distribution (e)

System BP F4TCNQ/BP

BP F4TCNQ

0V

1.0 V

2.0 V

0

0.004

-0.004

0.800 -0.911

0.778 -0.933

0.834 -0.940

NO2/BP

BP NO2

0.176

0.164

0.228

-0.251

-0.256

-0.248

MoO3/BP

BP MoO3

0.263 -0.268

0.239 -0.265

0.252 -0.259

13



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The calculated potential gradient is due to the differences between the two self-consistent charge densities under zero- and finite-biases, which exactly is the voltage potential. So to a certain extent, the electrostatic potential gradient at a plane above the molecule-absorbed region reflects the potential drop across the junction. We further analyzed the potential gradient by calculating the contribution from each atomic Mulliken charge in the central region with external electric field in Figure 1b. In generally, hole is a kind of carrier in opposite direction of electron and takes positive charge. All the directions of potential drops in Figure 1b are opposite to the applied electric field, which indicates the electronic transport behavior in monolayer phosphorene are hole-dominated whether before or after adsorbing molecules. The conjugated π-bond is delocalized on the whole monolayer phosphorene system, resulting in its symmetrical potential gradient. The charge transfer between absorbed molecules and phosphorene layer may cause the asymmetric potential gradient along monolayer phosphorene junction. The strongest charge transfer between F4TCNQ molecule and phosphorene layer leads to the extremely asymmetric potential gradient of F4TCNQ/BP system at the bias of 1.0 V (Figure 1b). Obviously, the potential drop occurs outside of the region where F4TCNQ is absorbed, showing that the region without the absorbed F4TCNQ is where the scattering of carriers takes place. When the bias rises to 2.0 V, a higher potential stage is distributed at the same region where F4TCNQ is absorbed. However, in the case of NO2/BP and MoO3/BP systems, the potential drops much faster in the regions that contain absorbed MoO3 and NO2 14


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molecules as compared to that in F4TCNQ/BP system, whether at the bias of 1.0 V or 2.0 V. Since the potential gradient can partially give atomic-level information about the electronic transport barrier,46 hole can transport more efficiently between electrode in the devices with slower potential drop. Therefore, the relatively flat potential gradient in F4TCNQ/BP system demonstrates that the adsorption of F4TCNQ can significantly improve the electron transport efficiency in monolayer phosphorene devices. 28 21


14

MoO 3/BP

b

F4TCNQ/BP NO2/BP

12

MoO3/BP

9

Ron/off

a Current (µ A)

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7

6 3

0 0.0

0.5

1.0

1.5

2.0

0 0.0

0.5

Bias (V)

1.0

1.5

2.0

Bias (V)

Figure 2. (a) I−V curves of pure monolayer phosphorene and phosphorene with the adsorption of F4TCNQ, NO2, and MoO3 molecules, respectively. (b) Current after/before ratio before and after the adsorption of molecules as a function of the applied bias.

Molecular adsorption not only affects the static electronic properties of monolayer phosphorene, but also leads to the change of its dynamic electronic transport behavior. To explicitly evaluate the performance of monolayer phosphorene device, we employed the NEGF method to calculate the transport transmission and the corresponding current-voltage (I-V) relation before and after molecular adsorption,

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which allows monitoring the conductivity change. The current was calculated by the Landauer–Büttiker formula47:

I=

%

&% , −

!

− −

" #$ '

(3)

where f is the Fermi function, µL and µR are the electrochemical potentials of the left and right electrodes, respectively, and the difference in the electrochemical potentials is given by eV with the applied bias voltage V, i.e., µL(V) = µL(0)-eV/2 and µR (V) =

µR(0)+eV/2. I-V curves of monolayer phosphorene devices before and after adsorbing molecules are given in Figure 2a. The current of monolayer phosphorene device increases exponentially with the bias. When F4TCNQ or NO2 is adsorbed on phosphorene layer, the current increases dramatically relative to that of pure monolayer phosphorene, and the current of F4TCNQ/BP device is much larger than that of NO2/BP at the bias higher than 1.0 V. At the bias of 2.0 V, the current of F4TCNQ/BP device has reached 25 µA that is nearly one time more than that of pure monolayer phosphorene, while the current of NO2/BP has merely increased by 40%. In contrast, in the case of the adsorption of MoO3, it declines slightly. By comparing the I-V curves of phosphorene devices before and after adsorbing molecules, the adsorption of F4TCNQ improves the electronic properties of monolayer phosphorene most significantly, revealing its great potential as surface dopant to enhance the transport properties of monolayer phosphorene. Generally, the sensitivity of monolayer phosphorene device to molecular adsorption, 16


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i.e., the current after/before ratios R(V) = Iafter/Ibefore for the adsorption of F4TCNQ and NO2, first increase between 0.4 V and 0.8 V and then decreases when the bias is higher than 0.8 V (Figure 2b). At the bias higher than 1.0 V, monolayer phosphorene device is much more sensitive to the adsorption of F4TCNQ than to the other two kinds of adsorbed molecules. However, opposite to the after/before status of F4TCNQ/BP and NO2/BP systems, the adsorption of MoO3 results in a slight decrease of current, giving rise to an after/before ratio less than 1. In addition, the current after/before ratio of MoO3/BP system is not as sensitive to the bias as F4TCNQ/BP and NO2/BP systems. When the bias reaches 2.0 V, the after/before ratio for the adsorption of F4TCNQ has reached 1.90, which is 1.32 times of the adsorption of NO2 and 2.44 times of the adsorption of MoO3. This result indicates that the species of the adsorbed molecule plays an important role in determining the performance of monolayer phosphorene device. The electronic transport of pure monolayer phosphorene is solely attributed to the conjugated P-P bond. However, in the monolayer phosphorene system with molecular adsorption, the case is more complicated, which is proposed to be caused by both the P-P π-conjugation and the charge transfer between the adsorbed molecule and phosphorene layer. It will be testified by the transmission and PDOS spectra as follows.

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3

b

BP F4TCNQ/BP

2

90

1

30

0 2

0 120

BP NO2/BP

-1

0 -2 BP MoO3/BP

3

BP NO2/BP

60

NO2 0 -60 90 BP MoO3/BP

60

2

MoO3

30

1 0 -2

BP F4TCNQ/BP F4TCNQ

60

DDOS (eV )

a

Transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1

0

1

Energy (eV)

2

0 -2

-1

0

1

2

Energy (eV)

Figure 3. (a) Transmission and (b) PDOS spectra of monolayer phosphorene systems with different adsorbed molecules at 2.0 V. Transmission spectra of monolayer phosphorene system (c) before and (d) after absorbing F4TCNQ molecule under various biases. The shadows denote the bias windows. The Fermi level of phosphorene electrodes is set to 0 eV.

To gain more insight into the dynamic electronic transport behaviors of monolayer phosphorene systems with different adsorbed molecules, we first analyze the transmission spectra which could support the proposals we put forward above. The transmission coefficient curve reflects the dynamic electronic feature of the molecule/BP system because the current is the integral of the transmission coefficient in an energy window around the Fermi level according to the Landauer–Büttiker

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theory. The transmission function T (E, V) of monolayer phosphorene device is the sum of transmission probabilities of all channels available at energy E under external bias voltage V48,

, = ()! * " , )" * + , #,

(4)

where GR and GA are the retarded and advanced Green’s functions, respectively, and coupling function ΓL and ΓR are the imaginary parts of the left and right self-energies. Self-energy depends on the surface Green’s functions of the electrode regions and comes from the nearest-neighbor interaction between the extended central region and the electrodes. Figure 3a shows the transmission spectra of monolayer phosphorene systems with different adsorbed molecules at 2.0 V. It is found that the nonzero transmission coefficient of pure monolayer phosphorene system emerges around the Fermi level with a width of 1.48 eV in the bias window (region in the shadow). The first transmission peaks below and above the Fermi level are initiated by the valence band maximum (VBM) and the conduction band minimum (CBM), respectively. Since the VBM peaks of monolayer phosphorene systems before and after molecular adsorption are closer to the Fermi level than the CBM peaks, their currents at 2.0 V are mostly contributed by the VBM orbital resonance, which indicates the p-type conductance of pure and molecule-adsorbed monolayer phosphorene. When F4TCNQ and NO2 molecules are adsorbed on phosphorene layer, the VBM peaks have been intensified and sharpened significantly. In the F4TCNQ/BP system, the integral area of the 19



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transmission peak in the bias window is much larger than those in the other systems. Hence its current is nearly 1 time higher than that of pure monolayer phosphorene system, 1.5 times higher than that of MoO3/BP system and 1/3 higher than that of NO2/BP system. The VBM peak changes of monolayer phosphorene before and after molecular adsorption can be ascribed to the following reasons. On the one hand, the sharpened VBM peak reveals that the adsorption of F4TCNQ reduces the delocalization degree of π-electrons in monolayer phosphorene. As a result, fewer channels distributed on phosphorene layer are provided for electron to transport between electrodes. This can be considered as the negative factor that suppresses the conductance of F4TCNQ/BP system. On the other hand, the VBM peak just located at the Fermi level is strengthened by more than 4 times from 0.16 to 0.68, which illustrates that the adsorbed F4TCNQ provides more channels for electrons to transport between electrodes efficiently. Electrons can transport through the adsorbed F4TCNQ molecule at the Fermi level. This can be considered as the positive factor that increases the conductance of F4TCNQ/BP system. Obviously, the positive factor has far exceeded the negative factor in F4TCNQ/BP system. Consequently, the electron transport efficiency is dramatically improved through the adsorbed F4TCNQ molecule. Furthermore, the hole transport efficiency in the opposite direction of electron is also promoted on phosphorene layer. Similar phenomenon can also be found in the transmission spectra of NO2/BP system. Nevertheless, different from the adsorption of F4TCNQ and NO2 molecules, the transmission integral area in the bias 20


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window is reduced by the adsorption of MoO3 and thus the current of MoO3/BP system is declined by 1/5. It is deduced that the adsorbed MoO3 molecule cannot provide enough channels to compensate the channel loss caused by the weakened delocalization of π-electrons in phosphorene layer. As one of the static characteristics, projected density of states (PDOS) represents the isolated energy levels that consist of energy levels shift and broadening caused by the molecular adsorption on monolayer phosphorene. PDOS can give us information on the number of basis orbitals in the central region that contribute to the eigenstate of the whole molecule/BP system at a certain energy E. PDOS of the combined system projected onto the basis orbitals of monolayer phosphorene system is calculated according to formula (5)49

, = -. |01 = 2∑ 45 65 7→: ; ∑ 5 < 4< 6< 7→:=, (5) 8

8

where 0 E is the eigenstate of the whole system, . E is the contribution of the basis orbitals of the central region to 0E, 6$ is the nonorthogonal basis set of the system, and ci and cj are expanding coefficients, respectively. The sum over i only runs over the basis orbitals of the central region, and the sum over j runs over all the basis orbitals of the whole system. Figure 3b shows the total and projected DOS of monolayer after molecular adsorption at 2.0 V. By comparison of the transmission spectra and PDOS shown in Figure 3a and 3b, the positions and heights of total DOS peaks (red lines in Figure 3b) for both the VBM and CBM of monolayer phosphorene in the bias window [-1.0 eV, 21



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1.0 eV] are significantly influenced upon the adsorption of F4TCNQ, NO2 or MoO3 molecules, which correspond well to their transmission spectra. The adsorption of these three kinds of molecules brings about several distinct states near the Fermi level, and the positions and heights of the projected DOS peaks of the isolated molecules are consistent to those in the total DOS spectra of molecule/BP systems. It is demonstrated that the higher densities of states located on the adsorbed molecules provide more channels for electrons to transport from the left electrode to the right electrode. The higher densities of states located on the adsorbed molecules can be attributed to the charge transfer between the adsorbed molecules and monolayer phosphorene. A lot of electrons are withdrawn from monolayer phosphorene to the adsorbed

molecules,

in

which

F4TCNQ

molecule

has

the

strongest

electron-withdrawing ability. In other words，holes are injected into phosphorene layer, realizing the p-type doping effect on monolayer phosphorene. It is noticed from Figure 2a that there is no current passing through monolayer phosphorene when the bias is lower than 0.6 V and then the current increases with the rise of bias voltage, whether before or after adsorbing molecules and whatever the species of the adsorbed molecules are. Figures 3c and 3d give us the transmission spectra of monolayer phosphorene system before and after adsorbing F4TCNQ at the bias from 0 V to 2.0 V that are helpful to explain the relationship between current and bias voltage. The transmission coefficient in the bias window is zero until the bias voltage is up to 0.6 V for both the pure monolayer phosphorene and F4TCNQ/BP 22


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systems. The bias voltage of 0.6 V corresponds to the value of the band gap of monolayer phosphorene. It seems that molecular adsorption has no effect on the intrinsic energy gap of pure monolayer phosphorene. When a bias voltage is applied, the VBM of the left electrode shifts upward with respect to the CBM of the right electrode. The current cannot flow through monolayer phosphorene system until VBM of the left electrode reaches CBM of the right electrode. In addition, the VBM peak height of monolayer phosphorene increases from 0 to 0.12 when the bias rises from 0.2 V to 1.8 V, which indicates that electrons can transport through the central region more efficiently under higher bias voltage. Meanwhile, the integral area under transmission peak in the bias window also increases with the rise of bias voltage. Accordingly, its current increases with the bias. When F4TCNQ is adsorbed on phosphorene layer, its VBM peak height has strikingly gone up to 0.64 at the bias of 1.8 V that is more than 5 times to that of monolayer phosphorene. Similar trends can also be observed in NO2/BP and MoO3/BP systems as shown in Figure S1. In comparison to the transmission spectra of the other systems, the F4TCNQ/BP system has the highest VBM peak and the largest integral area under transmission peak in the bias window at 1.8 V that is in accordance with the transmission spectra at 2.0 V as shown in Figure 3a. As a result, the F4TCNQ/BP system under a higher bias voltage has the highest current value of all. Therefore, the adsorption of electrophilic molecules can remarkably promotes the efficiency for electrons to transport from the

23



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left electrode to the right electrode and the efficiency for holes from the right electrode to the left electrode in monolayer phosphorene system.

Figure 4. Transmission pathways through monolayer phosphorene systems with different adsorbed molecules at an energy level of -0.52 eV under 2.0 V, respectively. The Fermi level of phosphorene electrodes is set to 0 eV. T is the corresponding transmission coefficient.

The components of transmission between pairs of atoms in molecule/BP systems, known as transmission pathway, can reflect the influence of adsorbed molecules on the electronic transport pathway in monolayer phosphorene system, since transmission function T(E,V) is the sum of transmission probabilities of all channels available at energy E under external bias V.50 Figure 4 shows the transmission pathways of monolayer phosphorene systems with different adsorbed molecules at 2.0

24


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V. The transmission pathways are represented by arrows distributed on the molecule/BP systems with a threshold of 0.12. They are normalized with the largest arrow in each plot being the same size, irrespective of the magnitude of the total transmission at -0.52 eV. The blue arrows give components of the transmission that are in the direction of the net current flow, thereby contributing to the current, whereas the red arrows give components in the opposite direction, thereby reducing the net current. The transmission probability of every channel is available from the arrow diameter. In the pure monolayer phosphorene system, electrons transport from the left electrode to the right electrode through P-P conjugated bond, however there are many components in the perpendicular direction of net current (purple) that lead to the scattering of electrons from the direction of the net current flow, thereby its current is suppressed. The binding between adsorbed molecule and phosphorene layer makes electron transfer from phosphorene layer to the adsorbed molecule, thus many arrows located on the molecule indicate that more channels are provided for electrons to transport through adsorbed molecules continuously. Meanwhile, the conjugated degree of P-P bond in the long π conjugation structure of monolayer phosphorene has been weakened by the binding between adsorbed molecule and phosphorene layer. As a result, the number of purple arrows is reduced, which represents the current loss caused by the electron scattering in the perpendicular direction of net current. There are two efficient electron transport routes in molecule/BP systems. The first one is 25



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through the P-P covalent bonds, the other one is through the adsorbed molecule. When the adsorbed molecule is changed from F4TCNQ to MoO3, the transmission coefficient through molecule/BP system has been reduced from 0.68 to 0.10, owing to the pathway loss located on the adsorbed molecule. Accordingly, the current response of monolayer phosphorene system to the adsorption of F4TCNQ is stronger than the other molecule/BP systems that we discussed herein.

Figure 5. Spatial distributions of LDOS for monolayer phosphorene systems with different adsorbed molecules at Fermi level under the bias of 2.0 V. The isosurface shown corresponds to 0.01 states/(Å3 eV).

The influence of charge transfer degree between adsorbed molecules and monolayer phosphorene on the electron transport routes through molecule/BP system can be seen clearly from the spatial distribution of local density of states (LDOS), that is the number of states per volume element and energy interval. As electrons can 26


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tunnel directly through the central region at Fermi level, we display the spatial distributions of LDOS for monolayer phosphorene systems before and after absorbing electrophilic molecules at Fermi level under 2.0 V (Figure 5), in which the isosurface corresponds to 0.01 states/(Å3 eV) and the color indicates the phase. In pure monolayer phosphorene system, states are projected on the P-P covalent bond, which demonstrates that the electron transport in monolayer phosphorene system is mainly attributed by the π electrons delocalized on the hexatomic ring structure. It has been confirmed that P-P covalent bond is the only pathway for electrons to transport between phosphorene electrodes that we put forward above (Figure 4). By comparing the density distributions of monolayer phosphorene systems before and after absorbing molecules, it is found that electrons can transport not only via the P-P covalent bonds, but also via the adsorbed molecule, which is most notable in F4TCNQ/BP system. When F4TCNQ is adsorbed on phosphorene layer, the density of states on P-P conjugated bonds has been dispersed to the adsorbed F4TCNQ molecule. It is indicated that the electron transport in F4TCNQ/BP system is more dependent on the route via F4TCNQ molecule, which greatly improves its total transmission coefficient. Consequently, the adsorption of F4TCNQ molecule can be applied in high performance devices based on monolayer phosphorene by surface charge transfer doping method.

27



b

BP VG = 0 V F4TCNQ/BP NO2/BP

28

IDS (µ A)

21

MoO3/BP 14 7 0 0.0

0.5

1.0

1.5

2.0

VDS(V) 40 30 20

d 48 V

VG = -1.0 V BP F4TCNQ/BP NO2/BP

DS


= 2.0 V

36

MoO3/BP

IDS (µ A)

c IDS (µA)

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10

MoO 3 /BP 24 12

0 0.0

0.5

1.0

1.5

2.0

-1.0

-0.5

VDS (V)

0.0

0.5

1.0

VG (V)

Figure 6. (a) Schematic structure of the simulated bottom-gate monolayer phosphorene FET. F4TCNQ molecule is adsorbed on monolayer phosphorene channel. Output characteristic curves of monolayer phosphorene FETs with different adsorbed molecules at gate voltage VG = 0 V (b) and 1.0 V (c), respectively. (d) Transfer characteristic curves of p-type monolayer phosphorene FETs with different adsorbed molecules at drain bias VDS = 2.0 V.

To explore the ballistic performance of monolayer phosphorene FETs with different adsorbed molecules, bottom-gate FET structures are modeled, as shown in Figure 6a and Figure S2. The geometry consists of a molecule-adsorbed monolayer phosphorene nanoribbon as the intrinsic channel material; the two end sides of which are assumed to be act as the source and drain contacts. The whole simulated region is embedded above a HfO2 (k = 29) dielectric layer with the thickness of 3.5 Å and a 28


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metallic gate electrode with the thickness of 0.5 Å is attached to the dielectrics below the intrinsic channel. This kind of bottom-gate geometry provides high gate-controlled electrostatics. Figure 6b and 6c are the calculated output characteristic curves of 38.8-Å channel monolayer phosphorene FETs with different adsorbed molecules at the gate voltages (VG) of 0 V and -1.0 V, respectively. The drain current density in pure monolayer phosphorene can reach 4.8×103 µA/µm at VG = 0V. It is ~44.3% higher than that before HfO2 dielectric layer is placed on the bottom of monolayer phosphorene, suggesting that the gate coupling induced by HfO2 dielectric layer with large dielectric constant contributes to the improved device performance. After the adsorption of F4TCNQ, the on-current of F4TCNQ/BP FET is remarkably improved to be 7.7×103 µA/µm that is ~58.7% higher than that of the pure monolayer phosphorene FET. In addition, the currents of monolayer phosphorene FET before and after molecular adsorption at VG = -1.0 V are all found to be higher than that at VG = 0 V, indicating the p-type conductance characteristics of monolayer phosphorene FETs. One of the most vital target for high performance FET is that Ion must be higher than 1.5×103 µA/µm.51 In our monolayer phosphorene FET, the drain current density goes well beyond the required value. Furthermore, the adsorption of F4TCNQ molecule dramatically improves the drain current density value. Hence, surface charge transfer doping by F4TCNQ molecule ensures monolayer phosphorene be applied as channel material in high-performance FET devices. 29



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Figure 6d shows the calculated transfer characteristic curves of p-type monolayer phosphorene FETs with different adsorbed molecules at source-drain bias VDS = 2.0 V. Quantitatively, the source-drain current (IDS) of pure monolayer phosphorene FET increases linearly with the decrease of gate voltage. When the gate voltage decreases to VG = -1.0 V, IDS of pure monolayer phosphorene FET has reached 3 times of the current at VG = 1.0 V. The lower current of MoO3/BP FET with respect to that of pure monolayer phosphorene can be attributed to the narrow width of monolayer phosphorene in the simulated model, which is so small that the scattering in the perpendicular direction to the boundary is not negligible. As shown in Figure S3 and S4, transmission spectra of monolayer phosphorene FETs with different adsorbed molecules at VDS = 2.0 V and different VG can give us reasons for their p-type transfer characteristic curves. With the decrease of gate voltage, the VBM peak is enhanced and moves closer to the Fermi level. This promotes the probabilities for electrons to transport between electrodes, implying that the decreased gate voltage leads to the strengthened coupling degree between adsorbed molecule and phosphorene layer. The above reasons can be further proved by comparing the spatial distribution of LDOS and transmission pathways of monolayer phosphorene FETs at VG = -1.0 V (Figure S5 and S6) and those without applied gate voltage (Figure 4 and Figure 5). By comparing the potential drops and charge transfers of monolayer phosphorene FETs at VG = -1.0 V (Figure S5 and Table S1) and that without the applied gate voltage (Figure 1 and Table 1), it is found that 30


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the directions of potential drops in monolayer FETs can be reversed by controlling the gate voltage, which is another evidence for the p-type conductance of monolayer phosphorene FETs. Carrier mobility is the average velocity of carriers per unit electric field, which is one of the most important parameters for a FET and reflects the moving speed of holes or electrons in semiconductor material. The hole carrier mobility (µ) of monolayer phosphorene FETs with different adsorbed molecules can be obtained by the following formula:

=

?@ !

ABC DEF

,

(6)

where gm is the slopes of transfer characteristic curves in the linear region. L = 38.84 Å is the effective channel length, W = 13.28 Å is the effective channel width. C0 is the capacitance per unit area that is given by GH = εH εJ/ℎ, in which ε = 29 is the dielectric constant of HfO2, ε0 = 8.854 × 10-12 F m-1 is the vacuum dielectric constant, h = 3.5 Å is the thickness of HfO2 layer, and S = 515.80 Å2 is the area of HfO2 layer. Accordingly, the hole carrier mobilities (µ) of monolayer phosphorene FETs with different adsorbed molecules are linearly dependent to the slopes of their transfer characteristic curves. The transfer characteristic curves of monolayer phosphorenes with different adsorbed molecules are nearly parallel to each other, which indicate that the hole mobility of monolayer phosphorene is independent on the adsorption of single electrophilic molecule. Alternatively, given the hole mobility of monolayer phosphorene with different adsorbed molecules are confirmed to be constant, their 31



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hole concentrations are linear dependent to their p-type conductivities. The results show that the hole concentration in F4TCNQ/BP FET at VDS = 2.0 V and VG = -1.0 V is almost 1.4 times of the value in NO2/BP FET and 6 times of that in pure monolayer phosphorene FET, which demonstrates that the adsorption of F4TCNQ has the strongest p-type doping effect on monolayer phosphorene. It should be noted that the p-type doping by the adsorption of NO2 is reversible due to the gradual desorption of NO2 molecules from the phosphorene layer once exposed to ambient air, which limits its application in p-type doped monolayer phosphorene FETs. So we come to the conclusion that it is more feasible for the adsorption of F4TCNQ to build high-performance electronic devices based on monolayer phosphorene by surface charge transfer doping method as it can enhance the hole concentration of monolayer phosphorene more significantly. To summarize, we have developed a new strategy to theoretically investigate the structural, static electronic and dynamic p-type transport properties of monolayer phosphorene with the adsorption of electrophilic molecules F4TCNQ, MoO3 and NO2, by using DFT combined with the NEGF method. The p-type conductance of monolayer phosphorene can be dramatically enhanced by the adsorption of F4TCNQ molecule, suggesting that surface charge transfer doping method can be applied to improve the performance of devices based on monolayer phosphorene. The static charge distribution and dynamic transmission spectra show that the charge transfer between adsorbed molecules and phosphorene layer results in the improved transport 32


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efficiency of holes in monolayer phosphorene. The output and transfer characteristics demonstrate that hole concentration in p-type monolayer phosphorene can be significantly increased by the adsorption of electrophilic molecules, especially the F4TCNQ molecule, and meanwhile the hole mobility keeps constant. The present theoretical method combining static and dynamic electronic properties studies together provides a systematical approach to study the surface charge transfer doping mechanism in monolayer phosphorene. The results shed light on the application of other 2D materials in high-performance nanodevices, besides of those based on monolayer phosphorene.

ASSOCIATED CONTENT

Supporting

Information

Available:

Transmission

spectra

of

monolayer

phosphorene with the adsorption of MoO3 and NO2 molecules under various biases. Schematic structures of the simulated bottom-gate NO2/BP and MoO3/BP FETs. Transmission spectra of monolayer phosphorene devices with different adsorbed molecules at VDS = 2.0 V under VG = 0 V and 1.0 V, respectively. Transmission spectra of monolayer phosphorene FETs with different adsorbed molecules at VDS = 2.0 V under various VG ranging from -1.0 V to 1.0 V. Potential drops of monolayer phosphorene FETs with different adsorbed molecules at VDS = 2.0 V and VG = -1.0 V. Spatial distributions of LDOS for monolayer phosphorene FETs with different adsorbed molecules at Fermi level under VDS = 2.0 V and VG = -1.0 V. Transmission

33



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pathways through monolayer phosphorene FETs with different adsorbed molecules at an energy level of -0.24 eV under VDS = 2.0 V and VG = -1.0 V. Optimized geometries of F4TCNQ/BP system without and with van der Waals correction. Charge changes (e) of monolayer phosphorene FETs with different adsorbed molecules at different drain voltages under the gate voltage VG = -1.0V.

AUTHOR INFORMATION

Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Nos. 2012CB932400, 2013CB933500), the Major Research Plan of the National Natural Science Foundation of China (Nos. 91233110, 91333208), the National Natural Science Foundation of China (Nos. 51172151, 21121091, 21273113 ), the National Science and Technology Support Project (No.2012BAF03B05), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This work was sponsored by the Qing Lan Project.

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Surface Charge Transfer Doping of Monolayer Phosphorene via

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