Although there are numerous optical spectroscopy techniques and methods that have been used to extract the fundamental bandgap of a semiconductor,most of them belong to one of these three approaches:(1)the excitonic a...Although there are numerous optical spectroscopy techniques and methods that have been used to extract the fundamental bandgap of a semiconductor,most of them belong to one of these three approaches:(1)the excitonic absorption,(2)modulation spectroscopy,and(3)the most widely used Tauc-plot.The excitonic absorption is based on a many-particle theory,which is physically the most correct approach,but requires more stringent crystalline quality and appropriate sample preparation and experimental implementation.The Tauc-plot is based on a single-particle theo⁃ry that neglects the many-electron effects.Modulation spectroscopy analyzes the spectroscopy features in the derivative spectrum,typically,of the reflectance and transmission under an external perturbation.Empirically,the bandgap ener⁃gy derived from the three approaches follow the order of E_(ex)>E_(MS)>E_(TP),where three transition energies are from exci⁃tonic absorption,modulation spectroscopy,and Tauc-plot,respectively.In principle,defining E_(g) as the single-elec⁃tron bandgap,we expect E_(g)>E_(ex),thus,E_(g)>E_(TP).In the literature,E_(TP) is often interpreted as E_(g),which is conceptual⁃ly problematic.However,in many cases,because the excitonic peaks are not readily identifiable,the inconsistency be⁃tween E_(g) and E_(TP) becomes invisible.In this brief review,real world examples are used(1)to illustrate how excitonic absorption features depend sensitively on the sample and measurement conditions;(2)to demonstrate the differences between E_(ex),E_(MS),and E_(TP) when they can be extracted simultaneously for one sample;and(3)to show how the popular⁃ly adopted Tauc-plot could lead to misleading results.Finally,it is pointed out that if the excitonic absorption is not ob⁃servable,the modulation spectroscopy can often yield a more useful and reasonable bandgap than Tauc-plot.展开更多
There is widespread and strong interest in trying to fabricate a metamaterial in which both the permittivity and permeability are equal to -1 in order to achieve sub-wavelength imaging. Several metamaterial constructs...There is widespread and strong interest in trying to fabricate a metamaterial in which both the permittivity and permeability are equal to -1 in order to achieve sub-wavelength imaging. Several metamaterial constructs have been proposed with varying degrees of success because of inherent losses, limited bandwidth and scattering from the abstracted circuit elements constituting the artificial material itself. A further limitation is the need to capture evanescent components from the object to be imaged that requires the lens to be located near the object. We have studied the underlying models and constraints that influence the design of a negative index lens and present this analysis as well as reviewing the opportunities. There are inevitable and well-known trade-offs between lens thickness, wavelength, dispersion and absorption. However, these can be characterized both numerically and experimentally, suggesting that a computational imaging approach to the recovery of sub-wavelength features might be effective. Depending on the specific details of the metamaterial employed for imaging, one can consider the data acquired to represent a set of coded apertures.展开更多
耕地中直接影响到粮食安全的是基本农田,实施耕地保护实质上就是实施基本农田保护。针对当前基本农田保护规划中存在的问题,以及本轮土地利用规划修编的特点和要求,考虑土地利用数据共享原则,对在基本农田保护规划中充分利用农用地分等...耕地中直接影响到粮食安全的是基本农田,实施耕地保护实质上就是实施基本农田保护。针对当前基本农田保护规划中存在的问题,以及本轮土地利用规划修编的特点和要求,考虑土地利用数据共享原则,对在基本农田保护规划中充分利用农用地分等成果进行了探讨,即在进行基本农田保护空间布局过程中,把农用地综合等作为一个重要因素加以考虑。在考虑粮食安全的基础上,再依据基本农田的内涵,建立了以基本农田保护规划决策模型为核心的技术方法。通过熵权系数法、理想点法与G IS技术的结合,基于M apbas ic编程并在M ap in fo平台上试验,建立了一套基本农田保护空间规划的技术路线,完成了从专业模型分析到规划图自动生成的全过程,实现了基本农田规划编制的决策支持和决策可视化,使规划成果更加科学合理。通过在济南市的实证,取得了令人满意的结果。展开更多
基金Supported by Bissell Distinguished Professor Endowment Fund at UNC-Charlotte。
文摘Although there are numerous optical spectroscopy techniques and methods that have been used to extract the fundamental bandgap of a semiconductor,most of them belong to one of these three approaches:(1)the excitonic absorption,(2)modulation spectroscopy,and(3)the most widely used Tauc-plot.The excitonic absorption is based on a many-particle theory,which is physically the most correct approach,but requires more stringent crystalline quality and appropriate sample preparation and experimental implementation.The Tauc-plot is based on a single-particle theo⁃ry that neglects the many-electron effects.Modulation spectroscopy analyzes the spectroscopy features in the derivative spectrum,typically,of the reflectance and transmission under an external perturbation.Empirically,the bandgap ener⁃gy derived from the three approaches follow the order of E_(ex)>E_(MS)>E_(TP),where three transition energies are from exci⁃tonic absorption,modulation spectroscopy,and Tauc-plot,respectively.In principle,defining E_(g) as the single-elec⁃tron bandgap,we expect E_(g)>E_(ex),thus,E_(g)>E_(TP).In the literature,E_(TP) is often interpreted as E_(g),which is conceptual⁃ly problematic.However,in many cases,because the excitonic peaks are not readily identifiable,the inconsistency be⁃tween E_(g) and E_(TP) becomes invisible.In this brief review,real world examples are used(1)to illustrate how excitonic absorption features depend sensitively on the sample and measurement conditions;(2)to demonstrate the differences between E_(ex),E_(MS),and E_(TP) when they can be extracted simultaneously for one sample;and(3)to show how the popular⁃ly adopted Tauc-plot could lead to misleading results.Finally,it is pointed out that if the excitonic absorption is not ob⁃servable,the modulation spectroscopy can often yield a more useful and reasonable bandgap than Tauc-plot.
文摘There is widespread and strong interest in trying to fabricate a metamaterial in which both the permittivity and permeability are equal to -1 in order to achieve sub-wavelength imaging. Several metamaterial constructs have been proposed with varying degrees of success because of inherent losses, limited bandwidth and scattering from the abstracted circuit elements constituting the artificial material itself. A further limitation is the need to capture evanescent components from the object to be imaged that requires the lens to be located near the object. We have studied the underlying models and constraints that influence the design of a negative index lens and present this analysis as well as reviewing the opportunities. There are inevitable and well-known trade-offs between lens thickness, wavelength, dispersion and absorption. However, these can be characterized both numerically and experimentally, suggesting that a computational imaging approach to the recovery of sub-wavelength features might be effective. Depending on the specific details of the metamaterial employed for imaging, one can consider the data acquired to represent a set of coded apertures.
文摘耕地中直接影响到粮食安全的是基本农田,实施耕地保护实质上就是实施基本农田保护。针对当前基本农田保护规划中存在的问题,以及本轮土地利用规划修编的特点和要求,考虑土地利用数据共享原则,对在基本农田保护规划中充分利用农用地分等成果进行了探讨,即在进行基本农田保护空间布局过程中,把农用地综合等作为一个重要因素加以考虑。在考虑粮食安全的基础上,再依据基本农田的内涵,建立了以基本农田保护规划决策模型为核心的技术方法。通过熵权系数法、理想点法与G IS技术的结合,基于M apbas ic编程并在M ap in fo平台上试验,建立了一套基本农田保护空间规划的技术路线,完成了从专业模型分析到规划图自动生成的全过程,实现了基本农田规划编制的决策支持和决策可视化,使规划成果更加科学合理。通过在济南市的实证,取得了令人满意的结果。
基金国家自然科学基金重点项目(90307017)国家自然科学基金(60676018,60806031)+3 种基金国家“九七三”重点基础研究发展规划项目(2005CB321701)教育部跨世纪优秀人才培养计划基金教育部高等学校博士学科点专项科研基金(20050246082)US National Science Foundation grants(CCF-0727791)
