原子力显微技术研究ZnO纳米棒的压电放电特性

在原子力显微镜的接触扫描模式下,研究了半导体ZnO纳米棒的压电放电特性.采用两步湿化学法制备沿c轴择优生长的ZnO纳米棒阵列;利用镀Pt探针接触扫描ZnO纳米棒获得峰值达120 pA电流脉冲,脉冲持续时间可达30 ms,电流脉冲与纳米棒的形貌存在对应关系.镀Pt探针与ZnO纳米棒接触形成肖特基二极管,I-V特性研究表明放电的ZnO纳米棒压电电势必须大于03 V,以驱动肖特基二极管并输出电流;放电时肖特基二极管的结电阻达吉欧(GΩ)量级,是影响压电电势输出的主要因 关键词： ZnO 纳米棒 压电放电 肖特基接触     

Fast-speed self-powered PEDOT: PSS/α-Ga2O3 nanorod array/FTO photodetector with solar-blind UV/visible dual-band photodetection

The $\alpha $-Ga$_{2}$O$_{3}$ nanorod array is grown on FTO by hydrothermal and annealing processes. And a self-powered PEDOT:PSS/$\alpha $-Ga$_{2}$O$_{3}$ nanorod array/FTO (PGF) photodetector has been demonstrated by spin coating PEDOT:PSS on the $\alpha $-Ga$_{2}$O$_{3}$ nanorod array. Successfully, the PGF photodetector shows solar-blind UV/visible dual-band photodetection. Our device possesses comparable solar-blind UV responsivity (0.18 mA/W at 235 nm) and much faster response speed (0.102 s) than most of the reported self-powered $\alpha $-Ga$_{2}$O$_{3}$ nanorod array solar-blind UV photodetectors. And it presents the featured and distinguished visible band photoresponse with a response speed of 0.136 s at 540 nm. The response time is also much faster than the other non-self-powered $\beta $-Ga$_{2}$O$_{3 }$ DUV/visible dual-band photodetectors due to the fast-speed separation of photogenerated carries by the built-in electric field in the depletion regions of PEDOT:PSS/$\alpha $-Ga$_{2}$O$_{3}$ heterojunction. The results herein may prove a promising way to realize fast-speed self-powered $\alpha $-Ga$_{2}$O$_{3}$ photodetectors with solar-blind UV/visible dual-band photodetection by simple processes for the applications of multiple-target tracking, imaging, machine vision and communication.     

Nonlinear optical properties of [（CH3）4N]Au（dmit）2 using Z-scan technique

The third-order optical nonlinearities of [（CH3）4N]Au（dmit）2 （dmit = 4,5-dithiolate-1,3-dithiole-2-thione） at 532 nm and 1064 nm are investigated using the Z-scan technique with pulses of picoseconds duration. The Z-scan spectra reveal a strong nonlinear absorption （reverse saturable absorption） and a negative nonlinear refraction at 532 nm. No nonlinear absorption is observed at 1064 nm. The molecular second-order hyperpolarizability γ for the [（CH3）4N]Au（dmit）2 molecule at 532nm is estimated to be as high as （2.1 ±0.1） × 10^-31 esu, which is nearly three times larger than that at 1064 nm. The mechanism responsible for the difference between the results is analysed. Nonlinear transmission measurements suggest that this material has potential applications in optical limiting.     

Annealing effect on structure and green emission of ZnO nanopowder by decomposing precursors

ZnO nanopowder is successfully synthesized by annealing the precursors in oxygen gas using the chemical precipitation method. Structural and optical properties of thus synthesized ZnO nanopowder are characterized by scanning electron microscopy (SEM) and photoluminescence (PL). The morphology of ZnO nanopowders evolves from nanorod to cobble as annealing temperature increases from 500 to 1000~\du, while spiral structures are observed in the samples annealed at 900 and 1000~\du. The PL spectra of ZnO nanopowder consist of largely green and yellow emission bands. The green emission from ZnO nanopowder depends strongly on the annealing temperature with a peak intensity at a temperature lower than 800~℃ while the yellow emission is associated with interstitial oxygen \rm O_\i.     

Two-photon optical characteristics of zinc oxide in bulk, low dimensional and nanoforms

We report the two-photon luminescence (TPL) and second-harmonic generation (SHG) characteristics of zinc oxide (ZnO) in ceramic, thick film and nanorod. Sintering, physical vapour deposition (PVD), and hydrothermal methods were used in preparing the three forms of samples, respectively. Wide-band luminescence in the visible wavelength was observed with green/yellow emission in ceramic and nano-crystal, and blue emission in the film. We attributed interstitial zinc and interstitial oxygen for the luminescence at blue and green bands. Images acquired by TPL and SHG microscopy are presented, interpretation is provided for granular structure exhibiting strong SHG at granular boundaries.     

ZnO nanorod arrays with tunable size and field emission properties on an ITO substrate achieved by an electrodeposition method

In the present work,vertically aligned ZnO nanorod arrays with tunable size are successfully synthesized on nonseeded ITO glass substrates by a simple electrodeposition method.The effect of growth conditions on the phase,morphology,and orientation of the products are studied in detail by X-ray diffraction(XRD),scanning electron microscopy(SEM),and transmission electron microscopy(TEM).It is observed that the as-prepared nanostructures exhibit a preferred orientation along c axis,and the size and density of the ZnO nanorod can be controlled by changing the concentration of ZnCl2.Field emission properties of the as-synthesized samples with different diameters are also studied,and the results show that the nanorod arrays with a smaller diameter and appropriate rod density exhibit better emission properties.The ZnO nanorod arrays show a potential application in field emitters.     

Influence of reducing anneal on the ferromagnetism in single crystalline Co-doped ZnO thin films

This paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on $a$-plane sapphire substrates by using molecular-beam epitaxy. The as-grown films show high resistivity and non-ferromagnetism at room temperature, while they become good conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour. The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase. Compared with weak ferromagnetism (0.16~$\mu _{\rm B}$/Co$^{2 + })$ in the Zn6110M, 7550P, 7280E, 7870Dhttp://cpb.iphy.ac.cn/CN/10.1088/1674-1056/19/5/056101https://cpb.iphy.ac.cn/CN/article/downloadArticleFile.do?attachType=PDF&id=111756Co-doped ZnO, diluted magnetic semiconductors, x-ray absorption fine structure, single crystalline thin filmsProject partially supported by National Science Foundation of China (Grant No.~10804017), National Science Foundation of Jiangsu Province of China (Grant No.~BK2007118), Research Fund for the Doctoral Program of Higher Education of China (Grant No.~20070286037), Cyanine-Project Foundation of Jiangsu Province of China (Grant No.~1107020060), Foundation for Climax Talents Plan in Six-Big Fields of Jiangsu Province of China (Grant No.~1107020070) and New Century Excellent Talents in University (NCET-05-0452).This paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on $a$-plane sapphire substrates by using molecular-beam epitaxy. The as-grown films show high resistivity and non-ferromagnetism at room temperature, while they become good conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour. The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase. Compared with weak ferromagnetism (0.16~$\mu _{\rm B}$/Co$^{2 + })$ in the Zn$_{0.95}$Co$_{0.05}$O single crystalline film with reducing annealing in the absence of Zn vapour, the films annealed in the reducing atmosphere with Zn vapour are found to have much stronger ferromagnetism (0.65~$\mu _{\rm B}$/Co$^{2 + })$ at room temperature. This experimental studies clearly indicate that Zn interstitials are more effective than oxygen vacancies to activate the high-temperature ferromagnetism in Co-doped ZnO films, and the corresponding ferromagnetic mechanism is discussed.Co-doped;ZnO;diluted;magnetic;semiconductors;x-ray;absorption;fine;structure;single;crystalline;thin;filmsThis paper reports that the high-quality Co-doped ZnO single crystalline films have been grown on a-plane sapphire substrates by using molecular-beam epitaxy.The as-grown films show high resistivity and non-ferromagnetism at room temperature,while they become more conductive and ferromagnetic after annealing in the reducing atmosphere either in the presence or absence of Zn vapour.The x-ray absorption studies indicate that all Co ions in these samples actually substituted into the ZnO lattice without formatting any detectable secondary phase.Compared with weak ferromagnetism(0.16 μB/Co2+) in the Zn0.95Co0.05O single crystalline film with reducing annealing in the absence of Zn vapour,the films annealed in the reducing atmosphere with Zn vapour are found to have much stronger ferromagnetism(0.65 μB/Co2+) at room temperature.This experimental studies clearly indicate that Zn interstitials are more effective than oxygen vacancies to activate the high-temperature ferromagnetism in Co-doped ZnO films,and the corresponding ferromagnetic mechanism is discussed.     

Preparation, structural and electrical properties of zinc oxide grown on silicon nanoporous pillar array

Polycrystalline thick film of zinc oxide (ZnO) is grown on a unique silicon substrate with a hierarchical structure, silicon nanoporous pillar array (Si-NPA), by using a vapour phase transport method. It is found that as-grown ZnO film is composed of closely packed ZnO crystallites with an average size of $\sim$10\,\mu$m. The film resistivity of ZnO/Si-NPA is measured to be $\sim$8.9\Omega\cdot$\,cm by the standard four probe method. The lengthwise $I$-$V$ curve of ZnO/Si-NPA heterostructure is measured. Theoretical analysis shows that the carrier transport across ZnO/Si-NPA heterojunction is dominated by two mechanisms, i.e. a thermionic process at high voltages and a quantum tunnelling process at low voltages.     

氧化锌纳米线的场发射

热蒸发法在硅基底上制备了任意取向的氧化锌纳米线阵列。经过热蒸发过程，硅基底表面覆盖了大量均匀分布的氧化锌岛，在这些岛上生长出了直径为几十纳米的非定向纳米线。出于实用考虑，基底周围的温度在制备过程中保持在500°C以下。从这些氧化锌纳米线获得了场发射。测得10μA/cm2所对应的开启场强为3.0V/μm。并且用透明阳极技术研究了发射中心分布。观察到场发射来自于整个样品表面。从这些结果可以看出氧化锌纳米线在平板显示器中有着巨大的应用潜力.     

Superhydrophobic surfaces via controlling the morphology of ZnO micro/nano complex structure

ZnO micro/nano complex structure films, including reticulate papillary nodes, petal-like and flake-hole, have been self-assembled by a hydrothermal technique at different temperatures without metal catalysts. The wettability of the above film surfaces was modified with a simple coating of heptadecafluorodecyltrimethoxy-silane in toluene. After modifying, the surface of ZnO film grown at 50~${^\circ}$C was converted from superhydrophilic with a water contact angle lower than 5$^{\circ}$ to superhydrophobic with a water contact angle of 165$^{\circ}$. Additionally, the surface of reticulate papillary nodes ZnO film grown at 100~${^\circ}$C had excellent superhydrophobicity, with a water contact angle of 173$^{\circ}$ and a sliding angle lower than 2$^{\circ}$. Furthermore, the water contact angle on the surface of petal-like and flake-hole ZnO films grown at 150~${^\circ}$C and 200~${^\circ}$C were found to be 140$^{\circ}$ and 120$^{\circ}$, respectively. The wettability for the samples was found to depend strongly on the surface morphology which results from the growth temperature.     

Improving light trapping and conversion efficiency of amorphous silicon solar cell by modified and randomly distributed ZnO nanorods

Three-dimensional （3D） nanostructures in thin film solar cells have attracted significant attention due to their appli- cations in enhancing light trapping. Enhanced light trapping can result in more effective absorption in solar cells, thus leading to higher short-circuit current density and conversion efficiency. We develop randomly distributed and modified ZnO nanorods, which are designed and fabricated by the following processes： the deposition of a ZnO seed layer on sub- strate with sputtering, the wet chemical etching of the seed layer to form isolated islands for nanorod growth, the chemical bath deposition of the ZnO nanorods, and the sputtering deposition of a thin Al-doped ZnO （ZnO：Al） layer to improve the ZnO/Si interface. Solar cells employing the modified ZnO nanorod substrate show a considerable increase in solar energy conversion efficiency.     

Tunable phase-stabilized infrared parametric laser source

This paper reports that the tunable self-phase-stabilized infrared laser pulses have been generated from a two-stage optical parametric amplifier. With an 800 nm pump source, the output idler pulses are tunable from 1.3 μm to 2.3 μm, and the maximum output energy of the idler pulses is higher than 1 mJ at 1.6 μm by using 6 mJ pump laser. A carrier-envelope phase fluctuation of ～ 0.15 rad (rms) for the idler pulses is measured for longer than one hour by using a home build f-to-2f interferometer.     

Central black hole mass determination for blazars

In this paper, we use a method to determine some basic parameters for the $\gamma$-ray loud blazars. The parameters include the central black mass ($M$), the boosting factor ($\delta$), the propagation angle (${\it {\it\Phi}}$), the distance along the axis to the site of the $\gamma$-ray production ($d$). A sample including 32 $\gamma$-ray loud blazars with available variability time scales has been used to discuss the above properties. In this method, the $\gamma$-ray energy, the emission size and the property of the accretion disc determine the absorption effect. If we take the intrinsic $\gamma$-ray luminosity to be $\lambda$ times the Eddington luminosity, i.e. $L_{\gamma}^{\rm in}=\lambda{L_{\rm Edd}}$, then we have the following results: the mass of the black hole is in the range of $(0.59-67.99)\times10^{7}M_{\odot} \ (\lambda=1.0)$ or $(0.90-104.13)\times10^{7}M_{\odot} \ (\lambda=0.1)$; the boosting factor ($\delta$) in the range of In this paper, we use a method to determine some basic parameters for the $\gamma$-ray loud blazars. The parameters include the central black mass ($M$), the boosting factor ($\delta$), the propagation angle (${\it {\it\Phi}}$), the distance along the axis to the site of the $\gamma$-ray production ($d$). A sample including 32 $\gamma$-ray loud blazars with available variability time scales has been used to discuss the above properties. In this method, the $\gamma$-ray energy, the emission size and the property of the accretion disc determine the absorption effect. If we take the intrinsic $\gamma$-ray luminosity to be $\lambda$ times the Eddington luminosity, i.e. $L_{\gamma}^{\rm in}=\lambda{L_{\rm Edd}}$, then we have the following results: the mass of the black hole is in the range of $(0.59-67.99)\times10^{7}M_{\odot} \ (\lambda=1.0)$ or $(0.90-104.13)\times10^{7}M_{\odot} \ (\lambda=0.1)$; the boosting factor ($\delta$) in the range of In this paper, we use a method to determine some basic parameters for the $\gamma$-ray loud blazars. The parameters include the central black mass ($M$), the boosting factor ($\delta$), the propagation angle (${\it {\it\Phi}}$), the distance along the axis to the site of the $\gamma$-ray production ($d$). A sample including 32 $\gamma$-ray loud blazars with available variability time scales has been used to discuss the above properties. In this method, the $\gamma$-ray energy, the emission size and the property of the accretion disc determine the absorption effect. If we take the intrinsic $\gamma$-ray luminosity to be $\lambda$ times the Eddington luminosity, i.e. $L_{\gamma}^{\rm in}=\lambda{L_{\rm Edd}}$, then we have the following results: the mass of the black hole is in the range of $(0.59-67.99)\times10^{7}M_{\odot} \ (\lambda=1.0)$ or $(0.90-104.13)\times10^{7}M_{\odot} \ (\lambda=0.1)$; the boosting factor ($\delta$) in the range of In this paper, we use a method to determine some basic parameters for the $\gamma$-ray loud blazars. The parameters include the central black mass ($M$), the boosting factor ($\delta$), the propagation angle (${\it {\it\Phi}}$), the distance along the axis to the site of the $\gamma$-ray production ($d$). A sample including 32 $\gamma$-ray loud blazars with available variability time scales has been used to discuss the above properties. In this method, the $\gamma$-ray energy, the emission size and the property of the accretion disc determine the absorption effect. If we take the intrinsic $\gamma$-ray luminosity to be $\lambda$ times the Eddington luminosity, i.e. $L_{\gamma}^{\rm in}=\lambda{L_{\rm Edd}}$, then we have the following results: the mass of the black hole is in the range of $(0.59-67.99)\times10^{7}M_{\odot} \ (\lambda=1.0)$ or $(0.90-104.13)\times10^{7}M_{\odot} \ (\lambda=0.1)$; the boosting factor ($\delta$) in the range of In this paper, we use a method to determine some basic parameters for the $\gamma$-ray loud blazars. The parameters include the central black mass ($M$), the boosting factor ($\delta$), the propagation angle (${\it {\it\Phi}}$), the distance along the axis to the site of the $\gamma$-ray production ($d$). A sample including 32 $\gamma$-ray loud blazars with available variability time scales has been used to discuss the above properties. In this method, the $\gamma$-ray energy, the emission size and the property of the accretion disc determine the absorption effect. If we take the intrinsic $\gamma$-ray luminosity to be $\lambda$ times the Eddington luminosity, i.e. $L_{\gamma}^{\rm in}=\lambda{L_{\rm Edd}}$, then we have the following results: the mass of the black hole is in the range of $(0.59-67.99)\times10^{7}M_{\odot} \ (\lambda=1.0)$ or $(0.90-104.13)\times10^{7}M_{\odot} \ (\lambda=0.1)$; the boosting factor ($\delta$) in the range of $0.16-2.09(\lambda=1.0)$ or $0.24-2.86\ (\lambda=0.1)$; the angle (${\it\Phi}$) in the range of $9.53^{\circ}-73.85^{\circ}\ (\lambda=1.0)$ or $7.36^{\circ}-68.89^{\circ}\ (\lambda=0.1)$; and the distance ($d/R_{\rm g}$) in the range of $22.39-609.36\ (\lambda=1.0)$ or $17.54-541.88\ (\lambda=0.1)$.     

Second-harmonic generation as a DNA malignancy indicator of prostate glandular epithelial cells

This paper first demonstrates second-harmonic generation (SHG) in the intact cell nucleus, which acts as an optical indicator of DNA malignancy in prostate glandular epithelial cells. Within a scanning region of $2.7~\mu$m$\times 2.7~\mu$m in cell nuclei, SHG signals produced from benign prostatic hyperplasia (BPH) and prostate carcinoma (PC) tissues (mouse model C57BL/6) have been investigated. Statistical analyses ($t$ test) of a total of 405 measurements (204 nuclei from BPH and 201 nuclei from PC) show that SHG signals from BPH and PC have a distinct difference ($p < 0.05)$, suggesting a potential optical method of revealing very early malignancy in prostate glandular epithelial cells based upon induced biochemical and/or biophysical modifications in DNA.     

Low-temperature heat transport of the zigzag spin-chain compound SrEr2O4

Low-temperature thermal conductivity ($\kappa$), as well as the magnetic properties and specific heat, are studied for the frustrated zigzag spin-chain material SrEr$_{2}$O$_{4}$ by using single-crystal samples. The specific heat data indicate the long-range antiferromagnetic transition at $\sim 0.73 $ K and the existence of strong magnetic fluctuations. The magnetizations at very low temperatures for magnetic field along the $c$ axis (spin chain direction) or the $a$ axis reveal the field-induced magnetic transitions. The $\kappa $ shows a strong dependence on magnetic field, applied along the $c$ axis or the $a$ axis, which is closely related to the magnetic transitions. Furthermore, high magnetic field induces a strong increase of $\kappa $. These results indicate that thermal conductivity along either the $c$ axis or the $a$ axis are mainly contributed by phonons, while magnetic excitations play a role of scattering phonons.     

The research on suspended ZnO nanowire field-effect transistor

This paper reports that a novel type of suspended ZnO nanowire field-effect transistors (FETs) were successfully fabricated using a photolithography process, and their electrical properties were characterized by I--V measurements. Single-crystalline ZnO nanowires were synthesized by a hydrothermal method, they were used as a suspended ZnO nanowire channel of back-gate field-effect transistors (FET). The fabricated suspended nanowire FETs showed a p-channel depletion mode, exhibited high on--off current ratio of ～10⁵. When V_DS=2.5 V, the peak transconductances of the suspended FETs were 0.396 μS, the oxide capacitance was found to be 1.547 fF, the pinch-off voltage V_TH was about 0.6 V, the electron mobility was on average 50.17 cm²/Vs. The resistivity of the ZnO nanowire channel was estimated to be 0.96× 10²Ω cm at V_GS = 0 V. These characteristics revealed that the suspended nanowire FET fabricated by the photolithography process had excellent performance. Better contacts between the ZnO nanowire and metal electrodes could be improved through annealing and metal deposition using a focused ion beam.     

Linear polarization of radiation from ZnCdSe/ZnSe quantum wells induced by the anisotropic profile of interfaces

The low-temperature spectra of the exciton luminescence of the ZnCdSe/ZnSe quantum well grown in the [100] direction have been analyzed. It has been found that the observed radiation is linearly polarized in the well plane along either the [011] axis or the orthogonal $[0\bar 11]$ axis depending on the energy of exciting light. Polarization along the [011] axis is associated with the anisotropy of the geometric profiles of interfaces and corresponds to the polarization of excitons localized in the terraces elongated along the [011] axis, which are regions corresponding to an increase in the well thickness by a depth of one or several monolayers. The terraces appear due to the existence of growth steps on the interfaces. Anisotropy in the distribution of growth steps, i.e., a longer length of the steps along the [011] axis, is also responsible for the appearance of polarization along the $[0\bar 11]$ axis. Such a polarization appears due to the found specific channel of anisotropic exciton absorption. As a result of such absorption, free excitons with large wave vectors are excited. The excitation of excitons by light is indirect, because it involves additional processes of the elastic scattering of excitons on the interface growth steps. Prevailing scattering on an ensemble of longer growth steps (along the [011] axis) leads to the alignment of the wave vectors of excited excitons. A difference between the absorption probabilities for the polarizations of excitation along the [011] and $[0\bar 11]$ axes is caused by a difference between the probabilities of scattering on steps for excitons whose dipole moments and, correspondingly, the axes of the P-type wave functions for the holes involved in the process are oriented along and across the direction of the alignment of wave vectors of excitons.     

ZnO/Cu_2O异质结纳米阵列制备及光催化性能

利用湿化学法在FTO玻璃基底上制备了高度规整的ZnO纳米棒阵列(ZnO NRAs),以此为衬底,采用磁控溅射法在ZnO NRAs表面沉积Cu_2O薄膜。分别用X射线衍射仪、X射线光电子能谱、扫描电镜、光致光谱、紫外可见分光光度计和电化学工作站对样品的物相、形貌、吸收光谱、光电性能进行了表征,用甲基橙(MO)模拟有机物废水研究复合材料的光催化性能。结果表明:ZnO纳米棒为六方纤锌矿结构,其直径约为80~100 nm,长约2~3μm,棒间距约100~120 nm。立方晶系的Cu_2O颗粒直径约为100~300 nm,形成致密膜层并紧密覆盖在ZnO NRAs表面上,构成ZnO/Cu_2O异质结纳米阵列(ZnO/Cu_2O HNRAs)结构。与纯ZnO NRAs和Cu_2O相比,ZnO/Cu_2O HNRAs在可见光范围内的吸收显著增强,吸收波长向可见光方向偏移。ZnO/Cu_2O HNRAs的载流子传递界面的电荷转移速度快,有效促进了光生电子和空穴的分离。在紫外-可见光照射65 min后,ZnO/Cu_2O HNRAs的降解效率为94%,分别是纯ZnO NRAs和Cu_2O的18倍和1.7倍。     

Controlling the orientation of ZnO nanorod arrays using TiO<Subscript>2</Subscript> thin film templates dip-coated by sol–gel

The oriented ZnO nanorod arrays have been synthesized on a silicon wafer that coated with TiO₂ films by aqueous chemical method. The morphologies, phase structure and the photoluminescence (PL) properties of the as-obtained product were investigated by field-emission scanning electron microscopy (FE-SEM), X-ray diffractometer (XRD), transmission electron microscope (TEM) and PL spectrum. The nanorods were about 100 nm in diameter and more than 1 μm in length, which possessed wurtzite structure with a c axis growth direction. The room-temperature PL measurement of the nanorod arrays showed strong ultraviolet emission. The effect of the crystal structure and the thickness of TiO₂ films on the morphologies of ZnO nanostructures were investigated. It was found that the rutile TiO₂ films were appropriate to the oriented growth of ZnO nanorod arrays in comparison with anatase TiO₂ films. Moreover, flakelike ZnO nanostructures were obtained with increasing the thickness of anatase TiO₂ films.     

Calculating the dielectric anisotropy of nematic liquid crystals: a reinvestigation of the Maier——Meier theory

This paper investigates the average dielectric permittivity (\overline ε ) in the Maier--Meier theory for calculating the dielectric anisotropy (Δε) of nematic liquid crystals. For the reason that \overline ε of nematics has the same expression as the dielectric permittivity of the isotropic state, the Onsager equation for isotropic dielectric was used to calculate it. The computed \overline ε shows reasonable agreement with the results of the numerical methods used in the literature. Molecular parameters, such as the polarizability and its anisotropy, the dipole moment and its angle with the molecular long axis, were taken from semi-empirical quantum chemistry (MOCPAC/AM1) modeling. The calculated values of Δε according to the Maier--Meier equation are in good agreement with the experimental results for the investigated compounds having different core structures and polar substituents.