Cheng-Quan Li

Spectroscopic consideration of the surface potential built across phthalocyanine thin films on a metal electrode

The nonlinear optical properties of tert-butyl phthalocyanine copper Langmuir-Blodgett (CuttbPc LB) films and vacuum-evaporated phthalocyanine copper (CuPc) films deposited on a metal surface were investigated by second-harmonic generation (SHG) spectroscopy. At the organic/metal interface, a space charge field is formed due to the presence of excess charge injected from a metal electrode to the organic layer. Since the Pc molecule has D4h symmetry, an inversion center is present and the optical SH process is not allowed under the electric-dipole approximation. However, the space charge field at the interface directly influences the symmetric structure of the electrons in the Pc molecule. We investigated the contributions of the surface potential to the SHG using Pc LB and vacuum-evaporated films deposited on aluminum (Al) and gold (Au) metal electrodes, where a distinctive difference in the spectrum for the Pc films on the Al and Au surfaces was observed. The contribution of the surface potential was revealed based on the resonant conditions of the SH process, taking into account the electric-quadrupole transition and dc-field-induced electric-dipole transition.

Equilibrium and kinetics: Water confined in carbon nanotubes as one-dimensional lattice gas

We present a simple one-dimensional lattice gas model, which describes very well the equilibrium and kinetic behaviors of water confined in a thin carbon nanotube found in an atomistic molecular dynamics simulation [G. Hummer, J. C. Rasaiah, and J. P. Noworyta, Nature (London), 414, 188 (2001)]. The model parameters correspond to various physical interactions and can be calculated or estimated by using statistical mechanics. Then, the roles of all interactions in the water filling, emptying, and transporting processes are clearly understood. Our results indicate that the interaction from the water molecules outside the nanotube plays a key role in these processes and the interaction can be simply treated as an average effect of the bulk water.

Second harmonic generation from copper-tetratert-butyl-phthalocyanine Langmuir–Blodgett film/metal interface: Electric quadrupole or electric field induced second harmonic generation effect?

It has been found that a high electric field is created at the interface between copper-tetratert-butyl-phthalocyanine (CuttbPc) Langmuir–Blodgett (LB) film/metal due to the exchanged charges. The symmetry of the CuttbPc molecule is broken by this space charge induced electric field (SCIEF), and optical second harmonic (SH) signal is generated from the CuttbPc LB film/metal interface. A theoretical expression of the reflected SH signal intensity is derived using a four-layer model with the SCIEF being treated as a mean field. The origins of the SHG are considered as the quadrupole effect and the electric field induced SHG (EFISHG) due to the SCIEF. On the basis of experimental results and the theoretical analysis, the nonlinear susceptibilities of these two effects are calculated. It is found that at the wavelength of incident fundamental light around 1100 nm the electric quadrupole effect makes a main contribution to the SHG from the interface, while at the wavelength around 1200 nm it is attributed to t...

Analysis of Potential Distribution and Current-Voltage Characteristic in Polyimide Langmuir-Blodgett Films

The current–voltage (I–V) characteristics of Au electrode (M)–polyimide (PI) Langmuir–Blodgett (LB) film (I)–Al electrode (M)-MIM element are analyzed, taking into account the interfacial electrostatic phenomena and the presence of the interfacial electronic states. On the basis of the Richardson–Schottky model, the attractive force and potential energy created by a series of mirror image charges against both electrodes are calculated. A positive bias (Vex>0) applied to an Au–PI LB film–Al element gives rise to the injection of electrons from the Au electrode into the lowest unoccupied molecular orbitals (LUMO) of PI LB film, whereas a negative bias (Vex<0) causes the injection from Al electrode. The potential distributions across the PI LB film and the potential barrier height under the condition of various externally applied voltages (Vex) are calculated, assuming the presence of surface states at the metal/film interface. It is determined that the calculation results of the I–V characteristics can predict the experimental results.

Static electric field effect in the second harmonic generation from phthalocyanine film/metal electrode

The effect of static electric field on the second harmonic generation (SHG) from copper phthalocyanine (CuPc) was investigated using metal/CuPc/metal structure. An external DC voltage was applied to the sample to induce an electric dipole which allowed second order polarization in the centrosymmetric molecular system (CuPc film). The second harmonic generation (SHG) spectra and its intensity vs. the external bias at fixed fundamental frequency were investigated. Considering the induced electric quadrupole transition and the electric field induced dipole transition, the SHG generated from the CuPc film was attributed to the electric quadrupole effect and the electric field induced SHG (EFISHG). A quadratic relation between the SHG intensity and the external applied bias was observed. It was in accordance with theoretical analysis.

Study of Second Harmonic Generation from Copper-tetra-tert-butyl-phthalocyanine Langmüir-Blodgett Film/Metal Interface

The second harmonic generation (SHG) from the copper-tetra-tert-butyl-phthalocyanine (CuttbPc) Langmuir-Blodgett (LB) film deposited on a glass substrate coated with evaporated metal electrode is investigated. Besides the electric quadrupole transition, the space charge induced electric field at the LB film/metal electrode interface breaks the centrosymmetry of the CuttbPc molecule and contributes to the generation of SH radiation. Based on the experimental results and theoretical analysis, the nonlinear susceptibilities of these two effects are calculated. It is found that at the incident fundamental light wavelength of 1100 nm, the electric quadrupole effect makes a main contribution to the SHG, while at the wavelength of around 1200 nm, it is attributed to the electric field induced SHG (EFISHG).

Space charge distribution and capacitance-voltage characteristics of metal/polyimide Langmuir-Blodgett film/metal structure

The capacitance-voltage (C-V) characteristics of the metal/polyimide (PI) Langmuir-Blodgett (LB) film/metal structure are analyzed, taking into account the interfacial electrostatic phenomena and the presence of the interfacial electronic states. The capacitance (/spl Cscr/) and the additional relative capacitance [(/spl Cscr/-/spl Cscr//sub 0/)//spl Cscr//sub 0/] at various applied external voltages (V/sub ex/) are calculated, under the assumption that the electron acceptor density of states (DOS) at the interfaces has a Gaussian profile centered at the energy level of the lowest unoccupied molecular orbital (LUMO). The results reveal that when a positive bias (V/sub ex/>0) is applied to a metal/insulator (PI LB film)/metal (MIM) element, the capacitance and the additional relative capacitance decrease as the thickness of PI LB film increases, while a negative bias (V/sub ex/<0) gives rise to slight changes of the capacitance and the additional relative capacitance. In addition, it is found that the density of space charge also experiences slight changes as the applied external bias increases. The calculated results of the C-V characteristics of the MIM element show good agreement with the experimental results.

Space Charge Distribution and Capacitance-Voltage Characteristics of Metal/Polyimide Langmuir-Blodgett Film/Metal Structure

The capacitance?voltage (C?V) characteristics of the metal/polyimide (PI) Langmuir-Blodgett (LB) film/metal structure are analyzed, taking into account the interfacial electrostatic phenomena and the presence of the interfacial electronic states. The capacitance (C) and the additional relative capacitance [(C-C0)/C0] at various applied external voltages (Vex) are calculated, under the assumption that the electron acceptor density of states (DOS) at the interfaces has a Gaussian profile centered at the energy level of the lowest unoccupied molecular orbitals (LUMO). The results reveal that when a positive bias (Vex>0) is applied to a metal/insulator (PI LB film)/metal (MIM) element, the capacitance and the additional relative capacitance decrease as the thickness of PI LB film increases, while a negative bias (Vex<0) gives rise to slight changes of the capacitance and the additional relative capacitance. In addition, it is found that the density of space charge also experiences slight changes as the applied external bias increases. The calculated results of the C?V characteristics of the MIM element show good agreement with the experimental results.

Electric-Field-Induced Second Harmonic Generation from Vacuum-Evaporated Metallophthalocyanine Film/Metal Electrode Interface

The optical second harmonic generation (SHG) was used to study the redistribution of space charges, which was caused by photoillumination, at the nanosized interface between metal electrode and phthalocyanine (Pc) film. The redistribution of space charges was observed using the SHG spectra, where the space-charge-induced electric field (SCIEF) broke the centrosymmetry of the Pc molecule and gave rise to another effective second order nonlinear polarization aside from the one induced by the second order electric quadrupole. Furthermore, an external bias was applied to the sandwiched metal-copper phthalocyanine (CuPc)-meal sample to induce the second order nonlinear polarization. The quadratic relationship between SHG intensity and the external applied bias was observed. On the basis of the four-layer model, the electric-field-induced SHG (EFISHG) process was analyzed. The theoretical results showed good agreement with the experimental data.

Study of SHG from CuttbPc LB film/metal Interface

l.Introduction As one of the most primitive nonlinear optical phenomena, second harmonic generation (SHG) has been investigated for half a century and it has become a very useful spectroscopic tool in the study of surface and interface. Under the electric dipole approximation, it was theoretically shown that SH signal cannot be generated in a centrosymmetric system. However, the generation of SH signal from the centrosymmetric molecules such as CuPc[l, 2] has been detected. In our recent experiments, an enhancement of SH signal was detected from the CuPc derivative, CuttbPc LB films deposited on metal coated glass substrate[3]. According to Yamada et al.l4l, the SHG from CuPc film deposited on glass substrate was attributed to the resonant enhancement of the electric-quadrupole transition based on an orientation gas model. On the other hand, based on our surface potential studies, it is found that excessive charges are transferred from metal to the CuttbPc LB film until a thermodynamic equilibrium is established at the interface between LB film and metal[5,6]. Due to the presence of these exchanged charges, a very high electric field with an order of 107-108 V/m is created. The symmetry of CuttbPc LB film is broken due to the space charge induced electric field (SCIEF) and an electric field induced SH polarization is created at the space charge region of CuttbPc LB film.