Simulation on Field Enhanced Electron Transfer between the Interface

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Simulation on Field Enhanced Electron Transfer between the Interface of ZnO−Ag Nanocomposite Fengyu Sheng,† Chunxiang Xu,*,† Zhulin Jin,† Jiyuan Guo,† Shengjiang Fang,† Zengliang Shi,† and Jinlan Wang‡ †

State Key Laboratory of Bioelectronics, Department of Electronic Science and Engineering, Southeast University, Nanjing 210096, P. R. China ‡ Physics Department, Southeast University, Nanjing 211189, P. R. China ABSTRACT: By using the first-principles method, the localized electronic enhancement phenomenon of ZnO−Ag nanocomposite is investigated. It is revealed that the electron transfer between ZnO and Ag cluster results in the localized surface plasmon resonance effect in view of infrared spectra. First, the charge density distribution is simulated, it demonstrates the displacement of electron charge cloud of Ag cluster after attaching Ag to ZnO, it also indicates the charge transfer between Ag and ZnO cluster. The electronic structures are significantly modulated by an applied alternating electric field, which leads to the oscillation of electron charge cloud to generate the localized surface plasmon resonance. The distributions of local electron states in the vicinity of the Fermi level reveal the enhancement of several modes in the infrared spectra of the neutral and charged ZnO−Ag nanocomposite in electric field. Then, the investigation on the Hirshfeld charges and electrostatic potential derived charges demonstrates that the charge transport happens between Ag cluster and ZnO cluster and becomes much more obvious with the increased electric field. Finally, the simulation on the infrared vibrational spectra exhibits the significantly enhanced spectra modes of ZnO and demonstrates the influence of electric field direction and intensity on the spectral structure after the Ag cluster is attached. nanoscale.14−17 Recently, ZnO nanostructures decorated by Ag nanoparticles show superior optical and electrical properties compared to pure ZnO or Ag because the plasmon peak of Ag is close to the intrinsic photoluminescence of ZnO and result in the resonance. Ag nanoparticles have be in situ grown on ZnO nanocrystals by chemical methods and further applied to enhance the photocatalytic performance.18 The surface electron transfer leads to the enhancement of UV light emission from Ag/Au−ZnO composites.19 One of the most important applications of LSPR is SERS, which is the resent research hotspot. There are two mechanisms to explain the overall SERS effect: the electromagnetism (EM) mechanism and the charge transfer (CT) mechanism, and the two enhancements almost take place simultaneously.20 The EM mechanism is broadly reported by designing different arrays. While the CT mechanism is uncontrollable because it is difficult from experiment to gain deep insight into the enhancement associated with the CT mechanism.21 Although there are a large number of reports on ZnO−Ag nanocomposites, the nature of enhancement mechanism of semiconductor-based SERS substrate is still ambiguous.

1. INTRODUCTION Recently, the noble metal and semiconductor nanocomposites based on the surface plasmon resonance (SPR) have drawn considerable interests, due to the improvement of optical, electronic, magnetic, and chemical properties, and the promising application in catalysis, biomedicine, and photonics.1−3 It has been reported that the optical properties were significantly modified due to the SPR-induced electron transport and energy transfer from the metal surface to the surrounding semiconductor.4−8 For example, the silver-deposited TiO2 nanoparticles exhibited considerable surface enhanced Raman scattering (SERS) enhancement compared with that on the pure TiO2 surface.9 It is possible to excite the localized oscillation of free charges confined to the surface of nanoscaled metallic nanostructure by light illumination.10 This process is referred as localized surface plasmon resonance (LSPR), and results in enhancement of the electric field localized around the metal, thus enhance the surface enhanced Raman scattering (SERS) and light adsorption.1 Ag is referred as one of the hottest materials for LSPR because it possesses many unique properties such as bioaffinity, high chemical stability, and good charge storage capability.11−13 ZnO, as a semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, has attracted considerable attention because of the wide application in lasers, field effect transistors, solar cells, and field emitters in © XXXX American Chemical Society

Received: May 26, 2013 Revised: August 16, 2013

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opposite direction. The Ag cluster model is obtained by the annealing procedure based on Monte Carlo simulation and the ZnO cluster model is built as referred to in ref 22; both models are further optimized by using Dmol3 module. All calculations are performed by the spin-polarized DFT within the generalized-gradient approximation24 (GGA) implemented in the DMol3 package.25 The Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional26 is used without any symmetry constraints. Double numerical basis sets including d-polarization functions (DND) are used in the calculations. The convergence thresholds are set to 2 × 10−5 hartree for energy and 0.004 hartree/Å for force. The global cutoff radius is set to 4.0 Å. A thermal smearing of 0.005 hartree is employed to make the electronic structure convergent. The accuracy of this PBE/DND scheme has been assessed via testing calculations on the nanocomposites from low precision to high precision. Vibrational analysis of the model system in their equilibrium configurations has been made to ensure that there are no imaginary frequencies corresponding to the saddle points on the potential energy surface (PES). The following electronic and optical properties are calculated on the basis of the optimized structure.

Because of the convenience and predictability of theoretical simulation, the first-principles calculation based on the density functional theory (DFT) was carried out to reveal the enhancement mechanism. With the help of electron density analysis module embedded in the software, the charge transfer phenomenon can be observed much more visualized, for a real insight to the nature of the mechanism. Here, the electron transfer between ZnO and Ag cluster and the resulted LSPR effect is revealed and discussed based on the electron density distribution analysis. First, the energy levels, formation energies, and band gaps of the neutral ZnO−Ag composite as a function of the applied electric field are calculated. Then the interaction and charge transfer between the Ag13 and (ZnO)15 cluster are investigated, the electron density distributions of states in the vicinity of Fermi level with and without an electric field are analyzed. The Hirshfeld charges and the electrostatic potential (ESP) derived charges are calculated in different fields. Finally, the infrared spectra of pure (ZnO)15 cluster and composite are compared, and the spectra enhancement of the composite are observed in different electric field. The infrared spectra demonstrate the change of electron density distributions of states and the related charge transfer between Ag and ZnO cluster.

3. RESULTS AND DISCUSSION 3.1. Energy Level and Formation Energy. For the individual subsystems, the simulation demonstrated that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy is −3.371 and −3.369 eV for Ag13 cluster and −5.93 and −3.815 eV for (ZnO)15 cluster, respectively. It is well-known that the free electrons in a metal will immigrate on the surface backward to the direction of the applied electric field, while a semiconductor are polarized to build up an electric field in the reverse direction. Here, the energy level, formation energy, work function, and HOMO−LUMO gap are calculated as a function of electric field for the neutral ZnO−Ag and listed in Table 1.

2. MODELS AND METHODS Here, a stable cluster (ZnO)15 was selected for our simulation, which have five ZnO benzene rings22 similar to a wurtzite nanorod. While Ag13, a regular icosahedra cluster with 20 (111) faces and each face formed by three Ag atoms, was selected as the smallest favorable structure due to the fewest surface broken bonds.23 The ZnO−Ag composite is formed by assembling Ag13 cluster onto the top of (ZnO)15 cluster. As shown in Figure 1a, the optimized ZnO−Ag composite forms a

Table 1. Calculated Electronic Parameters of the Neutral ZnO-Ag Composite Dependent on the Applied Electric Field (EF), Including the HOMO/LUMO Level (Eh/El), the Work Function (WF), the HOMO−LUMO Gap (Eg), and the Formation Energy (Ef) EF (V/Å)

Eh (eV)

El (eV)

WF (eV)

Eg (eV)

Ef (eV)

−0.5 −0.025 0 0.025 0.5

−16.875 −10.463 −3.891 2.415 8.664

−16.744 −10.337 −3.671 2.684 8.804

16.809 10.400 3.781 −2.549 −8.734

0.131 0.126 0.22 0.269 0.14

4.407 2.682 2.447 3.680 6.726

The formation energy of the composite is calculated by the equation Figure 1. Top view (a) and side view (b) of the optimized ZnO−Ag composite. A uniform electric field is applied along ZnO−Ag axis (marked as the blue arrow); the value of the field is positive with direction from ZnO to Ag and is negative in the opposite direction. The red, gray, and blue balls represent O, Zn, and Ag atoms, respectively.

Ef = (EZnO + EAg ) − Ecomp

(1)

where EZnO and EAg is the energy of the optimized and isolated ZnO and Ag cluster and Ecomp is the total energy of the optimized composite. The simulation reveals the formation energy of ZnO/Ag is 2.447 eV, as listed for the zero-field case in Table 1. The formation energy presents positive value and increases with the enhancing of the electric field. This indicates that the structure is energetically stable and becomes more and more stable with the increased electric field. The composite in a higher positive electric field exhibits higher formation energy

tube-like structure capped with spherical Ag13 cluster and has a face-to face contact, and the hexagonal outlines are almost parallel, shown in Figure 1a. A uniform electric field is applied along the ZnO−Ag axis, the value is positive when the direction is from ZnO to Ag displayed in Figure 1b and is negative in the B

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vacuum level from Table 1; it is clear that with the increasing electric field, the HOMO and LUMO levels are raised, while the work function is reduced. This indicates the same result that electrons in the ZnO−Ag composite are more likely to escape from the potential barrier. The work function even becomes negative when the field increases to the largest value of 0.5 V/Å. This means that the electrons in the composite easily escape as soon as a positive electric field is applied. When an appropriate molecule is attached to the composite, large numbers of electrons are ready to transfer from the composite to molecule and result in the enhancement of SERS of the molecule. The intensity of nontotally symmetric modes (such as the b2 mode in 4-aminothiophenol) is diagnostic of contributions to the enhancement from charge transfer transitions.27 3.2. Electron Density Distributions and Charge Analysis. To further investigate the charge transfer enhancement mechanism of the ZnO−Ag composite, the local electronic density distribution of states near Fermi level under the electric field ranging from −0.5 to 0.5 V/Å is calculated and displayed in Figure 3. Without an electric field, the electron density of the LUMO state is mainly localized at the surface of Ag cluster, while the distribution of HOMO states is mainly localized at the top of Ag cluster and the interface between ZnO and Ag clusters, as shown in Figure 3c. When a negative electric field is applied, the electric density distribution of HOMO and LUMO states just exchanged compared to the situation without electric field. This causes the drop of Fermi level, and the electrons in these composites are hard to escape to vacuum. When a positive electric field is applied, a significant change in the electron density distribution is observed. The electrons densities of LUMO distributed on the capped Ag cluster are dragged to the bottom of ZnO cluster and the electron densities of HOMO shift from the ZnO−Ag interface are brought to the surface of Ag under the strongest electric field of 0.5 V/Å. This make the energy levels of LUMO and HOMO shift to the vacuum, as displayed in the energy levels in Figure 4a. In this case, the small ZnO cluster plays an important role in field emission as a sharp emission site and amount of electrons accumulated on the site are ready to escape to vacuum. In order to have an insight to the electron transfer phenomenon and its contribution to field enhancement, the charge accumulation in the capped Ag cluster in different electric field is calculated. The Hirshfeld and ESP charges analysis are recommended because they yield chemically meaningful charges.28 As shown in Figure 4b, the two curves are with the same variation trend. The electron accumulation is sensitive to the applied electric field and increase almost linearly with the increase of field. The negative charges in a negative field means the electrons transfer from ZnO to Ag, while the positive charge in the positive field shows the transfer of electrons from Ag to ZnO. It is familiar to us that on the effect of an applied positive electric field, the free electrons in Ag tend to transfer to the opposite site of field and inject to ZnO. When a negative field is applied, the bounding electrons are polarized on the surface of ZnO. When the field is big enough, the bound electrons are stimulated and tunnel into Ag cluster. It is obvious from above that the electron cloud of Ag oscillates linearly with the linear variation of electric field. It is deducible that with a time-resolved electric field the electron cloud of Ag can oscillate collectively with a resonant frequency,

because the Ag and O atoms at the interface of the composite form a strong ionic bond, and the bond is strengthened with the increase of field. The electron occupation of Ag and O atoms in the calculated composite is listed in Table 2. It is Table 2. Electron Occupation of O and Ag Atoms in the Composite, where N Denotes the Principal Quantum Number and L Represents the Angular Quantum Number; All Electrons Are Calculated without Electric Field element

N

L

occupation

energy (eV)

O

1 2 2 4 4 4 5

0 0 1 0 1 2 0

2 2 4 2 6 10 1

−514.176 −23.855 −8.976 −95.578 −58.173 −6.917 −3.712

Ag

obvious that the energy levels of O-2p and Ag-5s orbitals are close, which indicates a localized bond formed in this energy domain. Also we can see an orbital hybridization in Ag-5s and O-2p orbitals. As in Figure 2, the density of states (DOS) of

Figure 2. Partial density of state (PDOS) of Ag-5s orbital and O-2p orbital; the red line represents the 2p orbital of O atom, and the black line represents the 5s orbital of Ag atom.

Ag-5s and O-2p are displayed. The peak values between −6 to 0 eV are in common. This gives more proof about the localized bond between Ag and O. Moreover, the one occupation of Ag-5s and four occupation of O-2p demonstrate that Ag is electrophobic and ready to donate electrons, while oxygen is electrophilic and ready to reaccept electrons, thus electrons transfer from the Ag-5s to O-2p orbital. They partially form Ag−O bonds with the bond length of 2.35 Å, which is close to the bond length in bulk Ag2O (2.05 Å). The two components combine into the complex through chemical adsorption. With the increase of electric field, the electron transfer is large and results in strong bond energy. So the chemical adsorption between the two components becomes stronger. The band gap of ZnO−Ag (0.22 eV) is largely condensed compared to ZnO cluster (2.01 eV) due to the introduction of states in the middle of the gap originated from the Ag cluster. Since the states in the vicinity of Fermi level are responsible for the field emission, we mainly focus on the electronic states on both sides of the Fermi level, which is denoted as the HOMO and LUMO states. The Fermi level is defined as the midgap of HOMO and LUMO and the work function is defined as the energy difference between the Fermi level and C

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Figure 3. Side views of the localized electronic density distribution of LUMO (upper) and HOMO (down) states for neutral ZnO−Ag composite with −0.5 to 0.5 V/Å electric field, and the isovalue is 0.03 e/Å3. The wave function is a negative value for the yellow and a positive value for the blue.

Figure 4. (a) Energy levels of ZnO−Ag for a neutral state with 0, 025, and 0.5 V/Å electric fields. The Fermi levels are drawn by dashed lines. (b) Hirshfeld and ESP charge population analysis of Ag cluster in different electric fields. (c) Scheme of band structure of ZnO and Ag with uniform Fermi level by electron transfer between ZnO and Ag.

cluster is higher than that of ZnO cluster, as a result, the work function of Ag cluster is smaller than that of ZnO cluster. Hence, the electrons in the conduction band of Ag (considered as the LUMO levels in our model) is much easier to transfer to

referred to as LSPR. The charge transfer between ZnO and Ag is the source of LSPR in this case. The charge transfer process can also be explained by the band theory, illustrated in Figure 4c. Since the Fermi level of Ag D

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as mentioned above. As we know, the electrons in the molecule would oscillate on the effect of applied alternating electric field. Here, we simulate the infrared spectra of the composite in different electric fields, exhibited in Figure 6. Most of the peaks

the conduction band of ZnO during the formation of ZnO−Ag composite, shifting the Fermi level of ZnO in the composite to a higher value and that of Ag to a lower value, and finally results in an equivalent Fermi level. All the data marked in Figure 4c are calculated in this work. Optical properties are modified by LSPR induced from the charge transfer. In ref 19, with the intensity of UV-light emission centered at 352 nm, the Ag−ZnO nanocomposite becomes much higher than that of the ZnO because the efficiency of the electron transfer from metal to semiconductor becomes high. In ref 27, the blue shift of the plasmon peak of gold in the Au/ZnO suggests that the electron density of Au was increased. The increase of electron density of Au nanoparticles may be due to transfer of the electrons from the ZnO to Au. Here, we investigate LSPR effect resulting from the charge transfer from a new point of view, which is the infrared spectra. 3.3. Infrared Vibration Enhancement. The infrared adsorption spectra are simulated by the vibration analysis module for an insight into the charge transfer contribution to the field enhancement. As displayed in Figure 5, the infrared

Figure 6. Infrared spectra of ZnO−Ag composite in different applied electric fields, corresponding to −0.5 (a), 0 (b), and 0.5 V/Å (c) from bottom to top.

in the spectra present a red shift in a positive field compared to that without field because much more electrons in Ag transfer to the interface of ZnO. While for the case of a negative field, the boundaries of the spectra is much broadened. It is imagined when an alternating electric field is applied, the electrons oscillate strongly and yield the surface plasmon and further give rise to the extra increase of field enhancement factor. It is mentioned above that the electrons in ZnO−Ag easily escape in an applied positive electric field. Here, the composite was charged with one positive charge and regarded as one electron escaped. Then the infrared spectra of the charged ZnO−Ag composite and the corresponding electric density distributions of states in an applied electric field are displayed in Figure 7. The dominant bands of positive charged composite in the range of 200−300 cm−1 are greatly suppressed, and the weak bands from 350 to 600 cm−1 are greatly enhanced for several times compared to the neutral one. This is due to the electrons of the charged composite rearranged in the electric field, seen in Figure 7b,c, and more electrons transfering from Ag to ZnO, changing from 1.386 Hirshfeld charges in the neutral composite to 1.845 ones in the charged composite. Once an electron is extracted from the composite, more electrons are accumulated in the end of ZnO and ready to escape on the effect of the electric field. The ZnO−Ag composite is a good candidate for SERS substrate. It is breathtaking to suggest that the Raman signals of the molecule are likely to be enhanced if a functionalized molecule is interconnected to this composite because the amount of electrons would transfer from the surface of ZnO to the molecule. Richter et al. observed the degree of chargetransfer for ZnO−PATP−Ag compounds by using surfaceenhanced Raman spectroscop , and found that the smaller size had an increased CT degree.19 Deng et al. reported that Ag nanocluster-decorated ZnO nanowire arrays, with a standard Raman analyte R6G, show morphology-dependent electromagnetic scattering SERS enhancement factors of 30−50 times for a 150 nm NC−ZnO NW array relative to a 10 nm NC−ZnO NW array with 532 nm excitation.29 The correlative

Figure 5. Infrared spectra of pure ZnO cluster (red curve) and ZnO− Ag composite (black curve).

spectra of pure ZnO and the composite are compared. The predominant bands in the spectrum of pure ZnO are located in 38, 431, 449, 514, 588, and 606 cm−1, which are assigned to the bending vibration of O−Zn−O in ZnO cluster, whereas the strong bands of the ZnO−Ag composite observed at 390, 429, and 451 cm−1 are almost unchanged. The bands around 500 cm−1 are broadened, and the intensity is reduced. The modes at 567 cm−1 are observed strongly, enhanced for more than one order magnification, while the mode around 588 cm−1 is suppressed and 598 cm−1 have a slight red-shift compared to the pure one. This is related to the different electron distribution between pure ZnO and the composite. For pure ZnO, it is neutral and the electrons distribute averagely in the cluster. While in the composite, electrons transfer from Ag to ZnO and make the ZnO cluster electronegative, the electron density distributions are mainly on the interface of ZnO and Ag E

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much more localized and enhanced. The composite investigated in this work gives us a good consult to design the SERS substrate and suggests further work should focus on such hybrid nanostructures.

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AUTHOR INFORMATION

Corresponding Author

*(C.X.) E-mail: [email protected]. Tel: 86-025-83790755. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work is supported by ″973″ Program (Grant Nos. 2011CB302004), NSFC (61275054), and MOE (20110092130006). REFERENCES

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Figure 7. (a) Infrared spectra for neutral and charged ZnO−Ag composite with 0.5 V/Å electric field. Side views of the localized electron density distributions of LUMO and HOMO states for the neutral composite (b) and the charged one (c). The isovalue is 0.03 e/Å3. The wave function is a negative value for the yellow and a positive value for the blue.

work is being carried out in our group, and we suggest future experiments give more attention to the ZnO−Ag composite.

4. CONCLUSIONS In this work, the electrons transfer between the metal and semiconductor are simulated, and the enhancement of electron resonance is observed and results in the LSPR that is demonstrated from the infrared spectra. (ZnO)15 cluster capped with Ag cluster with magic number of 13 atoms is optimized using the first-principles method. Our results show that the Ag and O atoms in the contact form strong ionic bonds and the composite becomes much more stable with the increased applied electric field. Electrons in Ag cluster transfer into ZnO cluster when forming the composite, which leads to the modification of the infrared vibration spectra of the composite and the enhancement of several vibrational modes. While in different fields, different electron density distributions are observed that lead to the change of IR spectra. The composite with electrons accumulated in ZnO cluster has a broader range of spectra, and that with electrons depleted in ZnO has a red shift of spectra. The electrons of the composite within a large electric field easily escape, and the distribution of electric density of the charged composite is still localized in the ZnO cluster. The spectra of the positive charged composite are F

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