Graphene used in THz ST

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Graphene NEGF examples

e the publication of research in the mid-1980s describing the formation of freeform graphene there has been an enormous growth in interest in the material. Graphene is of interest to the semiconductor industry because of the high electron mobility exhibited by the material and, as it is planar, it is compatible with silicon technology. When patterned into nanoribbons graphene can be made into regions that are semiconducting or conducting and even into entire circuits. Graphene nanoribbons can also be used to form the channel of a MOSFET. This thesis describes numerical simulations undertaken on devices formed from graphene. The energy band structure of graphene and graphene nanoribbons is obtained using nearest-neighbour and third nearest-neighbour interactions within a tight binding model. A comparison of the current-voltage characteristics of MOS structures formed on graphene nanoribbons and carbon nanotubes suggests that the nanoribbon devices may be better for switching applications. Conductivities of graphene nanoribbons and junctions formed from them were obtained using a nonequilibrium Green’s function formulation. The effects of defects and strain on these systems were also studied using this technique. Advancements were made when the self-energies used within the nonequilibrium Green’s function were obtained from an iterative scheme including third nearestneighbour interactions. An important result of this work is that accurate simulations of graphene based devices should include third nearest-neighbour interactions within the tight binding model of the energy band structure.

可重构太赫兹石墨烯极化转换超表面

ｄｏｉ：１０．３９６９／ｊ．ｉｓｓｎ．１００３－３１１４．２０２２．０２．００４引用格式：司黎明，汤鹏程，吕昕．可重构太赫兹石墨烯极化转换超表面［Ｊ］．无线电通信技术，２０２２，４８（２）：２３３－２４０．［ＳＩＬｉｍｉｎｇ，ＴＡＮＧＰｅｎｇｃｈｅｎｇ，ＬＹＵＸｉｎ．ＲｅｃｏｎｆｉｇｕｒａｂｌｅＴｅｒａｈｅｒｔｚＧｒａｐｈｅｎｅＰｏｌａｒｉｚａｔｉｏｎＣｏｎｖｅｒｓｉｏｎＭｅｔａｓｕｒｆａｃｅ［Ｊ］．ＲａｄｉｏＣｏｍｍｕｎｉ⁃ｃａｔｉｏｎｓＴｅｃｈｎｏｌｏｇｙ，２０２２，４８（２）：２３３－２４０．］可重构太赫兹石墨烯极化转换超表面司黎明，汤鹏程，吕㊀昕（北京理工大学集成电路与电子学院／毫米波与太赫兹技术北京市重点实验室，北京１０００８１）摘㊀要：提出了一款基于石墨烯的太赫兹超表面单元，该单元能够对圆极化入射波起到极化转换的作用㊂通过调节石墨烯的化学势能，超表面单元的反射特性会发生改变㊂在此基础上，根据几何相位以及异常反射原理，利用超表面单元的相位调制特性，构建了３组不同相位梯度的超表面，并对不同化学势能条件下的近场特性进行了电磁仿真研究㊂更进一步，又利用超表面单元构成了１ｂｉｔ编码超表面㊂改变石墨烯的化学势能，可以对其散射远场方向图起到调控作用㊂该款基于石墨烯的太赫兹超表面单元为灵活调控太赫兹波提供了新的思路，未来有望应用到太赫兹频段上可重构智能超表面的构建当中㊂关键词：太赫兹；超表面；石墨烯；可重构；异常反射中图分类号：ＴＰ３９１．４㊀㊀㊀文献标志码：Ａ㊀㊀㊀开放科学（资源服务）标识码（ＯＳＩＤ）：文章编号：１００３－３１１４（２０２２）０２－０２３３－０８ＲｅｃｏｎｆｉｇｕｒａｂｌｅＴｅｒａｈｅｒｔｚＧｒａｐｈｅｎｅＰｏｌａｒｉｚａｔｉｏｎＣｏｎｖｅｒｓｉｏｎＭｅｔａｓｕｒｆａｃｅＳＩＬｉｍｉｎｇ，ＴＡＮＧＰｅｎｇｃｈｅｎｇ，ＬＹＵＸｉｎ（ＳｃｈｏｏｌｏｆＩｎｔｅｇｒａｔｅｄＣｉｒｃｕｉｔｓａｎｄＥｌｅｃｔｒｏｎｉｃｓ，ＢｅｉｊｉｎｇＩｎｓｔｉｔｕｔｅｏｆＴｅｃｈｎｏｌｏｇｙ，ＢｅｉｊｉｎｇＫｅｙＬａｂｏｒａｔｏｒｙｏｆＭｉｌｌｉｍｅｔｅｒＷａｖｅａｎｄＴｅｒａｈｅｒｔｚＴｅｃｈｎｏｌｏｇｙ，Ｂｅｉｊｉｎｇ１０００８１，Ｃｈｉｎａ）Ａｂｓｔｒａｃｔ：Ａｔｅｒａｈｅｒｔｚｍｅｔａｓｕｒｆａｃｅｕｎｉｔｂａｓｅｄｏｎｇｒａｐｈｅｎｅｉｓｐｒｏｐｏｓｅｄ，ｗｈｉｃｈｃａｎｒｅａｌｉｚｅｃｉｒｃｕｌａｒｐｏｌａｒｉｚａｔｉｏｎｃｏｎｖｅｒｓｉｏｎ．Ｂｙａｄｊｕｓ⁃ｔｉｎｇｔｈｅｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙｏｆｇｒａｐｈｅｎｅ，ｔｈｅｒｅｆｌｅｃｔｉｏｎｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆｔｈｅｍｅｔａｓｕｒｆａｃｅｕｎｉｔｗｉｌｌｃｈａｎｇｅ．Ｆｕｒｔｈｅｒｍｏｒｅ，ｂａｓｅｄｏｎｔｈｅｐｒｉｎｃｉｐｌｅｏｆｇｅｏｍｅｔｒｉｃｐｈａｓｅａｎｄａｎｏｍａｌｏｕｓｒｅｆｌｅｃｔｉｏｎ，ｔｈｒｅｅｇｒｏｕｐｓｏｆｍｅｔａｓｕｒｆａｃｅｓｗｉｔｈｄｉｆｆｅｒｅｎｔｐｈａｓｅｇｒａｄｉｅｎｔｓａｒｅｃｏｎｓｔｒｕｃｔｅｄ．Ｔｈｅｉｒｎｅａｒ⁃ｆｉｅｌｄｃｈａｒａｃｔｅｒｉｓｔｉｃｓｗｉｔｈｄｉｆｆｅｒｅｎｔｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙａｒｅｓｔｕｄｉｅｄｂｙｅｌｅｃｔｒｏｍａｇｎｅｔｉｃｓｉｍｕｌａｔｉｏｎｓ．Ｗｈａｔｓｍｏｒｅ，ａ１⁃ｂｉｔｃｏｄｅｄｍｅｔａｓｕｒｆａｃｅｉｓｃｏｎｓｔｒｕｃｔｅｄ．Ｂｙｃｈａｎｇｉｎｇｔｈｅｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙｏｆｇｒａｐｈｅｎｅ，ｔｈｅｆａｒ⁃ｆｉｅｌｄｐａｔｔｅｒｎｃｏｕｌｄｂｅｒｅｇｕｌａｔｅｄ．Ｔｈｉｓｇｒａｐｈｅｎｅｔｅｒａｈｅｒｔｚｍｅｔａｍａｔｅｒｉａｌｕｎｉｔｐｒｏｖｉｄｅｓａｎｅｗｉｄｅａｆｏｒｆｌｅｘｉｂｌｅｒｅｇｕｌａｔｉｏｎｏｆｔｅｒａｈｅｒｔｚｗａｖｅｓ，ａｎｄｉｓｅｘｐｅｃｔｅｄｔｏｂｅａｐｐｌｉｅｄｔｏｔｈｅｃｏｎｓｔｒｕｃｔｉｏｎｏｆｒｅｃｏｎｆｉｇｕｒａｂｌｅｉｎｔｅｌｌｉｇｅｎｔｓｕｒｆａｃｅｉｎｔｅｒａｈｅｒｔｚｂａｎｄｉｎｔｈｅｆｕｔｕｒｅ．Ｋｅｙｗｏｒｄｓ：ｔｅｒａｈｅｒｔｚ；ｍｅｔａｓｕｒｆａｃｅ；ｇｒａｐｈｅｎｅ；ｒｅｃｏｎｆｉｇｕｒａｂｌｅ；ａｎｏｍａｌｏｕｓｒｅｆｌｅｃｔｉｏｎ收稿日期：２０２２－０１－１９基金项目：国家重点基础研究项目（２０１９⁃ＪＣＪＱ⁃３４９）；国家重点研发计划（２０１８ＹＦＦ０２１２１０３）；国家自然科学基金（６１５２７８０５）；高等学校学科科研创新引智计划项目（Ｂ１４０１０）；北京理工大学国际合作项目（ＢＩＴＢＬＲ２０２００１４）ＦｏｕｎｄａｔｉｏｎＩｔｅｍ：ＮａｔｉｏｎａｌＰｒｏｇｒａｍｏｎＫｅｙＢａｓｉｃＲｅｓｅａｒｃｈＰｒｏｊｅｃｔ（２０１９⁃ＪＣＪＱ⁃３４９）：ＮａｔｉｏｎａｌＫｅｙＲｅｓｅａｒｃｈａｎｄＤｅｖｅｌｏｐｍｅｎｔＰｒｏｇｒａｍｏｆＣｈｉｎａ（２０１８ＹＦＦ０２１２１０３）；ＮａｔｉｏｎａｌＮａｔｕｒａｌＳｃｉｅｎｃｅＦｏｕｎｄａｔｉｏｎｏｆＣｈｉｎａ（６１５２７８０５）；ＨｉｇｈｅｒＥｄｕｃａｔｉｏｎＤｉｓｃｉｐｌｉｎｅＩｎｎｏｖａｔｉｏｎＰｒｏｊｅｃｔｏｆＣｈｉｎａ（Ｂ１４０１０）；ＩｎｔｅｒｎａｔｉｏｎａｌＣｏｏｐｅｒａｔｉｏｎＲｅｓｅａｒｃｈＢａｓｅＦｏｕｎｄａｔｉｏｎｏｆＢｅｉｊｉｎｇＩｎｓｔｉｔｕｔｅｏｆＴｅｃｈｎｏｌｏｇｙ（ＢＩＴＢＬＲ２０２００１４）０　引言电磁超表面是一种由亚波长单元结构周期排布在二维平面上，形成的平面型人工复合电磁材料［１－２］，可以通过改变其结构自由设计等效电磁参数，从而实现对电磁波灵活多样的调控㊂超表面的形式一旦确定，功能就被确定，不能实时可调㊂可重构智能超表面（ＲｅｃｏｎｆｉｇｕｒａｂｌｅＩｎｔｅｌｌｉｇｅｎｔＳｕｒｆａｃｅ，ＲＩＳ）的概念近来被提出［３］，能够解决这一问题㊂通过对表面中每个单元的相位㊁幅度或极化等电磁特性的单独控制，可以实现以编程的方式调控电磁波的效果，为物理电磁世界和信息科学世界之间的连接提供了接口㊂ＲＩＳ以其对无线电磁环境的调控，有望成为未来６Ｇ通信的关键技术㊂现阶段关于ＲＩＳ的研究处于起步，主要集中于信道模型的建立和理论分析［４－１１］，对结构的研究较少，并且大部分是关于低频段超材料结构的研究［１２－１５］㊂太赫兹通信是未来无线通信发展的必然趋势［１６］，寻找一种在太赫兹频段具有可调特性的结构具有重要现实意义㊂石墨烯作为一种新兴的晶格结构材料，拥有载流子迁移率高㊁机械强度大㊁可调谐性能强等优点，在太赫兹领域中存在巨大的应用潜力［１７－２２］㊂本文基于石墨烯的电导率可调特性，设计了一款在太赫兹频段动态可调的超表面单元，并利用其组成具有近场调控特性的异常反射超表面，以及远场方向图可重构的１ｂｉｔ编码超表面㊂１　基于石墨烯的超表面单元设计在没有外置偏置磁场的条件下，红外线频率以下波段上石墨烯的导电特性主要由带内跃迁产生，其等效电导率可以由简化的Ｋｕｂｏ公式［２３］计算得出：σ＝ｉω＋ｉτ－１ｅ２πћ２㊃２ｋＢＴ㊃ｌｎ２ｃｏｓｈμ２ｋＢＴ()[]，（１）式中，ω表示角频率，τ表示弛豫时间，ｅ表示基本电荷常数，ћ表示普朗克常数，ｋＢ表示玻尔兹曼常数，Ｔ表示温度，μ表示化学势能㊂超表面单元是构成超表面的基本结构㊂本文设计的超表面单元如图１所示，具体的结构参数如表１所示，长宽分别为ｐ，由两层高度分别为ｈ１和ｈ２的ＴＯＰＡＳ介质基板支撑㊂ＴＯＰＡＳ多聚物（相对介电常数εｒ为２．３４，损耗角正切为０．００００７）在太赫兹频段上能够保持稳定的介电常数，并且拥有较低的吸收损耗，是理想的太赫兹介质基板材料［２４］㊂最上层的金属贴片和最下层的金属地板均为Ａｇ（电导率为４．５６ˑ１０７Ｓ／ｍ）材质，金属贴片由金属环和工字形结构构成㊂两层介质基板中间为１０ｎｍ厚的石墨烯层，分别从石墨烯层与金属地板层引出电极，在两者之间设置偏置电路，通过控制偏置电压以实现对石墨烯化学势能的调控㊂该超表面单元的金属贴片结构沿ｕ轴和ｖ轴两方向上表现出各向异性的特点，对沿这两个方向极化垂直入射的电磁波将会产生不同的电磁响应㊂由于沿－ｚ轴方向垂直入射的圆极化电磁波可以分解成ｕ轴和ｖ轴两个方向上等幅度的极化分量，当这两个分量的反射相位相差奇数倍π时，将会发生极化转换的现象㊂（ａ）主视图㊀（ｂ）爆炸图图１㊀超表面单元结构示意图Ｆｉｇ．１㊀Ｓｃｈｅｍａｔｉｃｄｉａｇｒａｍｏｆｐｒｏｐｏｓｅｄｍｅｔａｓｕｒｆａｃｅｕｎｉｔ表１㊀结构参数Ｔａｂ．１㊀Ｇｅｏｍｅｔｒｉｃｐａｒａｍｅｔｅｒｓ参数ａｂｃｗｈ１ｈ２ｐ数值／μｍ５８３０４４６１０４７１２０在温度２７３Ｋ，石墨烯层化学势能为０ｅＶ，弛豫时间为０．１ｐｓ的条件下，对于沿－ｚ轴方向入射的右旋极化电磁波，所设计的超表面单元具备如图２所示的反射特性㊂图２㊀化学势能为０ｅＶ条件下，超表面单元反射幅度曲线Ｆｉｇ．２㊀Ｒｅｆｌｅｃｔｉｏｎａｍｐｌｉｔｕｄｅｏｆｐｒｏｐｏｓｅｄｍｅｔａｓｕｒｆａｃｅｗｉｔｈｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙｏｆ０ｅＶ一般情况下，右旋圆极化入射的电磁波经过金属面的反射，传输方向改变，电场的旋向不会变，反射波变为左旋圆极化波㊂而经过具备极化转换功能的超材料反射后，反射波依然为右旋圆极化波㊂在０．９４１．４８ＴＨｚ频段，右旋圆极化波反射率大于７５％，相对带宽４５％㊂极化转换率（ＰｏｌａｒｉｚａｔｉｏｎＣｏｎｖｅｒｓｉｏｎＲａｔｅ，ＰＣＲ）是衡量超表面单元极化转换性能的指标［２５］，对于右旋圆极化波入射的情况，极化转换率由可以下公式计算得出：ＰＣＲ＝Ｒ２ＲＲＲ２ＲＲ＋Ｒ２ＬＲ㊂（２）图３展示了该超表面单元的极化转换率，０．９３１．５ＴＨｚ频段之间极化转换率高于７０％㊂其中０．９７㊁１．２５和１．４７ＴＨｚ三个频点极化转换率接近于１００％，分别为９６％㊁９８％和９６％㊂图３㊀化学势能为０ｅＶ条件下，超表面单元的极化转换率Ｆｉｇ．３㊀ＰＣＲｏｆｐｒｏｐｏｓｅｄｍｅｔａｓｕｒｆａｃｅｗｉｔｈｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙｏｆ０ｅＶ因为本文所设计的超表面单元能够对于垂直入射的圆极化波能起到极化转换作用㊂根据几何相位原理［２６］，通过对超表面单元围绕ｚ轴旋转，可以实现对交叉圆极化波反射相位的调控㊂图４给出了围绕ｚ轴逆时针不同旋转角度下，右旋圆极化反射波（右旋圆极化波入射的情况下）的反射相位㊂当超表面单元旋转φ角度时，右旋圆极化波的反射相位产生接近２φ的稳定相位变化，与理论相符㊂图４㊀不同旋转角度条件下，超表面单元的反射相位曲线Ｆｉｇ．４㊀Ｒｅｆｌｅｃｔｉｏｎｐｈａｓｅｏｆｍｅｔａｓｕｒｆａｃｅｗｉｔｈｄｉｆｆｅｒｅｎｔｒｏｔａｔｉｏｎａｎｇｌｅｓ上述将超表面单元围绕ｚ轴旋转的方式仅仅是对单元贴片进行旋转，并没有保留超表面原本的晶格排列方式㊂超表面单元之间耦合特性的改变，会对反射性能产生影响㊂为了保持耦合特性的稳定，本文在该超表面单元的设计过程中引入了金属圆环㊂图５给出了不同旋转角度下，右旋圆极化波的反射系数，反射曲线几乎重合，具备极强的稳定性㊂由此可见金属圆环的引入确实带来了性能上的改善，超表面单元之间耦合特性基本上不会因为贴片的旋转而改变㊂图５㊀不同旋转角度条件下，超表面单元的反射幅度曲线Ｆｉｇ．５㊀Ｒｅｆｌｅｃｔｉｏｎａｍｐｌｉｔｕｄｅｏｆｍｅｔａｓｕｒｆａｃｅｗｉｔｈｄｉｆｆｅｒｅｎｔｒｏｔａｔｉｏｎａｎｇｌｅｓ２　超表面单元可调特性研究图６研究了不同化学势能条件下，对于沿－ｚ轴方向垂直入射的右旋圆极化波，超表面单元的右旋圆极化波反射特性㊂随着化学势能的提高，右旋圆极化波反射率不断降低㊂当化学势能大于０．８ｅＶ时，超表面单元工作频带内右旋圆极化反射率低于２０％㊂由此可见，通过控制石墨烯的化学势能，可以实现对超表面单元性能的调控㊂图６㊀不同化学势能条件下，右旋圆极化波反射幅度Ｆｉｇ．６㊀Ｒｅｆｌｅｃｔｉｏｎａｍｐｌｉｔｕｄｅｏｆｒｉｇｈｔ⁃ｈａｎｄｅｄｃｉｒｃｕｌａｒｌｙｐｏｌａｒｉｚｅｄｗａｖｅｗｉｔｈｄｉｆｆｅｒｅｎｔｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙ为了探究右旋圆极化反射场强度随化学势能的提高而降低的原因，本文对不同化学势能条件下该超表面单元的极化转换率和吸波率进行了研究㊂由图７可以看出，极化转换率会随化学势能的提高而急剧下降㊂当化学势能提升至０．３ｅＶ，０．６１．８ＴＨｚ整个频带内极化转换率均低于５０％㊂当化学势能高于０．７ｅＶ，该超表面单元几乎不具备极化转换的能力㊂化学势能的提高会使得石墨烯的电导率提高，石墨烯层相当于一层金属壁，破坏了金属贴片与金属地板之间的多重反射关系，因而极化转换率会降低㊂图７㊀不同化学势能条件下，超表面单元的极化转换率Ｆｉｇ．７㊀ＰＣＲｏｆｐｒｏｐｏｓｅｄｍｅｔａｓｕｒｆａｃｅｗｉｔｈｄｉｆｆｅｒｅｎｔｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙ因为该超表面单元具有极化转换特性，计算吸波率需要考虑交叉极化分量㊂吸波率（ＡｂｓｏｒｐｔｉｏｎＲａｔｅ，ＡＲ）可由以下公式［２７］计算得出：ＡＲ＝１－Ｒ２ＬＲ－Ｒ２ＲＲ㊂（３）图８展示了超表面单元的吸波特性㊂当化学势能为０ｅＶ时，超表面单元吸波率接近于０㊂当向石墨烯层施加偏置电压，化学势能不为０时，超表面单元开始具备吸波特性㊂化学势能增加的过程中，吸波带宽不断变宽㊂在１．６ＴＨｚ频点附近，出现了一段吸波峰，吸波率高于７０％，并且吸收峰会随化学势能的提高而发生蓝移㊂偏置电压使得石墨烯具备导电特性，电磁波会在石墨烯层上产生传导电流，进而产生焦耳损耗，超表面单元具备了吸波特性㊂图８㊀不同化学势能条件下，超表面单元的吸波率Ｆｉｇ．８㊀ＡＲｏｆｐｒｏｐｏｓｅｄｍｅｔａｓｕｒｆａｃｅｗｉｔｈｄｉｆｆｅｒｅｎｔｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙ从对超表面单元的极化转换率和吸波率的研究中，可以得知，石墨烯层对极化转换条件的破坏和电磁能量的吸收，共同导致右旋圆极化反射场的电场强度会随化学势能的提高而降低㊂３　梯度相位超表面由上文可知，通过旋转超表面单元，可以实现对圆极化反射波的相位调控㊂当具备梯度反射相位的超表面单元组合在一起时，超表面能够改变电磁波的正常传输方向㊂这种物理现象被称作为异常反射，其满足广义斯涅尔反射定律［２８］：ｎｒｓｉｎθｒ－ｎｉｓｉｎθｉ＝λ０２πｄφｄｘ，（４）式中，θｒ和θｉ分别为反射角和入射角（与超表面法线方向的夹角），ｎｉ为介质的折射率，ｄφ／ｄｘ表示单位长度上反射相位的变化㊂如果以等周期形式排列形成梯度相位超表面，广义斯涅尔反射公式可以进一步简化㊂通过以下公式能够计算出反射角度：θｒ＝ａｒｃｓｉｎλ０Ｌæèçöø÷，（５）式中，Ｌ表示周期长度㊂实际设计过程中Ｌ不能小于波长，否则将会产生表面波㊂由于超表面由具有离散反射相位的超表面单元构成，Ｌ＝ｎｐ，ｐ表示单元的晶格长度，ｎ表示一个相位变化周期的单元个数㊂本文考虑了ｎ＝３，４，６的３种排布情况，按照如图９所示的相位排布形式完成３０ˑ３０阵面规模的超表面构建，通过旋转单元完成反射相位沿ｘ轴方向的梯度离散变化㊂利用式（４）计算出理论反射角度分别为４４．９８ʎ㊁３１．３４ʎ和２０．３ʎ㊂在激励为沿－ｚ轴方向传播的右旋圆极化平面波的条件下进行仿真，图１０为１．２ＴＨｚ频点超表面附近右旋圆极化瞬时电场分布图㊂ｎ＝３，４，６的３种排布下，反射角度分别为４４．５ʎ㊁３１ʎ和２０ʎ，与理论值保持一致㊂此外，本文还研究了不同化学势能条件下，反射电磁波的近场分布情况㊂当化学势能为０ｅＶ，右旋圆极化电磁波的电场强度的幅度分别为１．２４Ｖ／ｍ㊁１．１５Ｖ／ｍ和１．０６Ｖ／ｍ；当化学势能提高到０．５ｅＶ，电场强度为０．３８Ｖ／ｍ㊁０．３７Ｖ／ｍ和０．４Ｖ／ｍ㊂化学势能继续提高，当化学势能为１ｅＶ时，反射场的强度接近０．２Ｖ／ｍ㊂（ａ）排布形式ｎ＝３㊀㊀（ｂ）排布形式ｎ＝４㊀㊀（ｃ）排布形式ｎ＝６（ｄ）局部结构ｎ＝３㊀㊀（ｅ）局部结构ｎ＝４㊀㊀（ｆ）局部结构ｎ＝６图９㊀梯度超表面相位排布形式及局部结构Ｆｉｇ．９㊀Ｐｈａｓｅａｒｒａｎｇｅｍｅｎｔａｎｄｌｏｃａｌｓｃｈｅｍａｔｉｃｄｉａｇｒａｍｏｆｔｈｅｍｅｔａｓｕｒｆａｃｅｗｉｔｈｇｒａｄｉｅｎｔｐｈａｓｅ（ａ）化学势能为０ｅＶ，ｎ＝３㊀（ｂ）化学势能为０．５ｅＶ，ｎ＝３㊀（ｃ）化学势能为１ｅＶ，ｎ＝３（ｄ）化学势能为０ｅＶ，ｎ＝４（ｅ）化学势能为０．５ｅＶ，ｎ＝４㊀（ｆ）化学势能为１ｅＶ，ｎ＝４（ｇ）化学势能为０ｅＶ，ｎ＝６（ｈ）化学势能为０．５ｅＶ，ｎ＝６㊀（ｉ）化学势能为１ｅＶ，ｎ＝６图１０㊀不同化学势能条件下，超表面近场特性Ｆｉｇ．１０㊀Ｎｅａｒｆｉｅｌｄｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆｐｒｏｐｏｓｅｄｍｅｔａｓｕｒｆａｃｅｗｉｔｈｄｉｆｆｅｒｅｎｔｃｈｅｍｉｃａｌｐｏｔｅｎｔｉａｌｅｎｅｒｇｙ㊀㊀由石墨烯构建的反射型各向异性超材料组成的梯度相位可重构超表面，可以通过控制石墨烯的化学势能，实现对反射波的幅度调控㊂偏置电压控制化学势能具有极快的响应速率［２９］，可满足时域幅度编码的要求㊂而通过改变超表面的反射相位梯度，又能够实现对电磁波反射角度的调控㊂两种调控方式相结合，为ＲＩＳ时空联合编码硬件平台的构建提供了一种新思路㊂４㊀散射方向图可重构超表面编码超表面调控电磁波的原理基于天线阵列原理，编码超表面由反射相位不同的超表面单元构成㊂对于垂直入射的平面波，编码超表面的散射远场函数［２７］为：㊀ｆ（θ，φ）＝ｆｅ（θ，φ）ðＮｍ＝１ðＮｎ＝１ｅｘｐ｛－ｉ｛ψ（ｍ，ｎ）＋ＫＤｓｉｎθ［（ｍ－１２）ｃｏｓφ＋（ｎ－１２）ｓｉｎφ］｝｝，（６）式中，θ为俯仰角，φ为方位角（超表面与地面平行），ψ（ｍ，ｎ）为每个单元的反射相位，Ｄ为单元间距，Ｋ为相位常数㊂由反射相位相差π的两种超表面单元，以数字０和１表示这两个单元，通过编码构成阵面，该超表面被称作为１ｂｉｔ编码超表面［３０］㊂图１１为本文设计的１ｂｉｔ编码超表面的结构示意图，由３０ˑ３０个超表面单元构成㊂通过将单元绕ｚ轴旋转９０ʎ的方式，实现π反射相位差㊂沿ｘ轴和ｙ轴方向每５个单元一组构成棋盘式排布阵面㊂（ａ）相位排布形式㊀（ｂ）局部结构图１１㊀１ｂｉｔ编码超表面相位排布形式及局部结构Ｆｉｇ．１１㊀Ｐｈａｓｅａｒｒａｎｇｅｍｅｎｔａｎｄｌｏｃａｌｓｃｈｅｍａｔｉｃｄｉａｇｒａｍｏｆｐｒｏｐｏｓｅｄ１ｂｉｔｃｏｄｅｄｍｅｔａｓｕｒｆａｃｅ图１２展示了０ｅＶ㊁０．２ｅＶ和１ｅＶ三组化学势能条件下，右旋圆极化平面波沿－ｚ轴垂直入射时，超表面的远场散射方向图㊂当化学势能为０ｅＶ时，散射方向图为与超表面法线方向夹角１７．８ʎ的四方向对称波束；化学势能提高到０．２ｅＶ，法线方向出现了第５个波束，而其余４个波束散射强度降低；当化学势能为１ｅＶ，超表面单元不具备极化转换的能力，仅剩下法向指向的左旋圆极化波束㊂（ａ）化学势能为０ｅＶ（ｂ）化学势能为０．２ｅＶ（ｃ）化学势能为１ｅＶ图１２㊀不同化学势能条件下，超表面远场散射图Ｆｉｇ．１２㊀Ｆａｒｆｉｅｌｄｓｃａｔｔｅｒｉｎｇｐａｔｔｅｒｎｓｏｆｐｒｏｐｏｓｅｄｍｅｔａｓｕｒｆａｃｅｗｉｔｈｄｉｆｆｅｒｅｎｔｃｈｅｍｉｃａｌｅｎｅｒｇｙ５㊀结束语本文基于石墨烯设计的超表面单元具备动态可调谐特性㊂通过改变石墨烯层的化学势能，在０．８１．６ＴＨｚ频段之间，交叉圆极化反射率能够在２０％８０％之间变化㊂利用该特性，本文将该单元应用于梯度相位超表面以及１ｂｉｔ编码超表面的设计中㊂在不同化学势能条件下，梯度相位超表面对应的反射波电场幅度动态可调，１ｂｉｔ编码超表面的远场散射方向图可以在单波束㊁４波束和５波束之间切换㊂数值仿真与理论计算结果一致性较好，证明了该设计方案的有效性㊂基于石墨烯的可重构超表面对太赫兹电磁波近场和远场具有优异调控性能，对于未来太赫兹频段上ＲＩＳ的构建具备启发性作用㊂参考文献［１］㊀ＨＯＬＬＯＷＡＹＣＬ，ＫＵＥＳＴＥＲＥＥ，ＧＯＲＤＯＮＪＡ，ｅｔａｌ．ＡｎＯｖｅｒｖｉｅｗｏｆｔｈｅＴｈｅｏｒｙａｎｄＡｐｐｌｉｃａｔｉｏｎｓｏｆＭｅｔａｓｕｒ⁃ｆａｃｅｓ：ＴｈｅＴｗｏ⁃ｄｉｍｅｎｓｉｏｎａｌＥｑｕｉｖａｌｅｎｔｓｏｆＭｅｔａｍａｔｅｒｉａｌｓ［Ｊ］．ＩＥＥＥＡｎｔｅｎｎａｓａｎｄＰｒｏｐａｇａｔｉｏｎＭａｇａｚｉｎｅ，２０１２，５４（２）：１０－３５．［２］㊀ＹＵＮ，ＣＡＰＡＳＳＯＦ．ＦｌａｔＯｐｔｉｃｓｗｉｔｈＤｅｓｉｇｎｅｒＭｅｔａｓｕｒｆａｃｅｓ［Ｊ］．ＮａｔｕｒｅＭａｔｅｒｉａｌｓ，２０１４，１３（２）：１３９－１５０．［３］㊀ＬＩＬＬ，ＣＵＩＴＪ，ＪＩＷ，ｅｔａｌ．ＥｌｅｃｔｒｏｍａｇｎｅｔｉｃＲｅｐｒｏｇｒａｍ⁃ｍａｂｌｅＣｏｄｉｎｇ⁃ｍｅｔａｓｕｒｆａｃｅＨｏｌｏｇｒａｍｓ［Ｊ］．ＮａｔｕｒｅＣｏｍｍｕ⁃ｎｉｃａｔｉｏｎｓ，２０１７，８（１）：１９７．［４］㊀ＷＡＮＧＪＨ，ＴＡＮＧＷＫ，ＨＡＮＹ，ｅｔａｌ．ＩｎｔｅｒｐｌａｙｂｅｔｗｅｅｎＲＩＳａｎｄＡＩｉｎＷｉｒｅｌｅｓｓＣｏｍｍｕｎｉｃａｔｉｏｎｓ：Ｆｕｎｄａｍｅｎｔａｌｓ，Ａｒｃｈｉｔｅｃｔｕｒｅｓ，Ａｐｐｌｉｃａｔｉｏｎｓ，ａｎｄＯｐｅｎＲｅｓｅａｒｃｈＰｒｏｂｌｅｍｓ［Ｊ］．ＩＥＥＥＪｏｕｒｎａｌｏｎＳｅｌｅｃｔｅｄＡｒｅａｓｉｎＣｏｍｍｕｎｉｃａｔｉｏｎｓ，２０２１，３９（８）：２２７１－２２８８．［５］㊀ＡＬＳＡＢＡＨＭ，ＮＡＳＥＲＭＡ，ＭＡＨＭＭＯＤＢＭ，ｅｔａｌ．６ＧＷｉｒｅｌｅｓｓＣｏｍｍｕｎｉｃａｔｉｏｎｓＮｅｔｗｏｒｋｓ：ＡＣｏｍｐｒｅｈｅｎｓｉｖｅＳｕｒｖｅｙ［Ｊ］．ＩＥＥＥＡｃｃｅｓｓ，２０２１，９：１４８１９１－１４８２４３．［６］㊀ＹＩＬＤＩＲＩＭＩ，ＵＹＲＵＳＡ，ＢＡＳＡＲＥ．ＭｏｄｅｌｉｎｇａｎｄＡｎａｌｙｓｉｓｏｆＲｅｃｏｎｆｉｇｕｒａｂｌｅＩｎｔｅｌｌｉｇｅｎｔＳｕｒｆａｃｅｓｆｏｒＩｎｄｏｏｒａｎｄＯｕｔｄｏｏｒＡｐｐｌｉｃａｔｉｏｎｓｉｎＦｕｔｕｒｅＷｉｒｅｌｅｓｓＮｅｔｗｏｒｋｓ［Ｊ］．ＩＥＥＥＴｒａｎｓａｃｔｉｏｎｓｏｎＣｏｍｍｕｎｉｃａｔｉｏｎｓ，２０２１，６９（２）：１２９０－１３０１．［７］㊀ＫＨＡＬＥＥＬＡ，ＢＡＳＡＲＥ．ＲｅｃｏｎｆｉｇｕｒａｂｌｅＩｎｔｅｌｌｉｇｅｎｔＳｕｒｆａｃｅ⁃ｅｍｐｏｗｅｒｅｄＭＩＭＯＳｙｓｔｅｍｓ［Ｊ］．ＩＥＥＥＳｙｓｔｅｍｓＪｏｕｒｎａｌ，２０２１，１５（３）：４３５８－４３６６．［８］㊀ＬＩＹ，ＸＩＯＮＧＪ，ＮＧＤＷＫ，ｅｔａｌ．ＥｎｅｒｇｙＥｆｆｉｃｉｅｎｃｙａｎｄＳｐｅｃｔｒａｌＥｆｆｉｃｉｅｎｃｙＴｒａｄｅｏｆｆｉｎＲｉｓ⁃ａｉｄｅｄＭｕｌｔｉｕｓｅｒＭＩＭＯＵｐｌｉｎｋＴｒａｎｓｍｉｓｓｉｏｎ［Ｊ］．ＩＥＥＥＴｒａｎｓａｃｔｉｏｎｓｏｎＳｉｇｎａｌＰｒｏｃｅｓｓｉｎｇ，２０２１，６９：１４０７－１４２１．［９］㊀ＱＩＡＮＸＷ，ＤＩＲＥＮＺＯＭ，ＬＩＵＪ，ｅｔａｌ．ＢｅａｍｆｏｒｍｉｎｇｔｈｒｏｕｇｈＲｅｃｏｎｆｉｇｕｒａｂｌｅＩｎｔｅｌｌｉｇｅｎｔＳｕｒｆａｃｅｓｉｎＳｉｎｇｌｅ⁃ｕｓｅｒＭｉｍｏＳｙｓｔｅｍｓ：ＳＮＲＤｉｓｔｒｉｂｕｔｉｏｎａｎｄＳｃａｌｉｎｇＬａｗｓｉｎｔｈｅＰｒｅｓｅｎｃｅｏｆＣｈａｎｎｅｌＦａｄｉｎｇａｎｄＰｈａｓｅＮｏｉｓｅ［Ｊ］．ＩＥＥＥＷｉｒｅｌｅｓｓＣｏｍｍｕｎｉｃａｔｉｏｎＬｅｔｔｅｒｓ，２０２１，１０（１）：７７－８１．［１０］ＰＥＲＯＶＩＣＮＳ，ＴＲＡＮＬＮ，ＤＩＲＥＮＺＯＭ，ｅｔａｌ．ＡｃｈｉｅｖａｂｌｅＲａｔｅＯｐｔｉｍｉｚａｔｉｏｎｆｏｒＭＩＭＯＳｙｓｔｅｍｓｗｉｔｈＲｅｃｏｎｆｉｇｕｒａｂｌｅＩｎｔｅｌｌｉｇｅｎｔＳｕｒｆａｃｅｓ［Ｊ］．ＩＥＥＥＴｒａｎｓａｃｔｉｏｎｓｏｎＷｉｒｅｌｅｓｓＣｏｍｍｕｎｉｃａｔｉｏｎｓ，２０２１，２０（６）：３８６５－３８８２．［１１］ＷＩＬＤＴ，ＢＲＡＵＮＶ，ＶＩＳＷＡＮＡＴＨＡＮＨ．ＪｏｉｎｔＤｅｓｉｇｎｏｆＣｏｍｍｕｎｉｃａｔｉｏｎａｎｄＳｅｎｓｉｎｇｆｏｒｂｅｙｏｎｄ５Ｇａｎｄ６ＧＳｙｓｔｅｍｓ［Ｊ］．ＩＥＥＥＡｃｃｅｓｓ，２０２１，９：３０８４５－３０８５７．［１２］ＨＥＪＨ，ＷＡＮＧＳＱ，ＬＩＸＷ，ｅｔａｌ．ＡＢｒｏａｄｂａｎｄＲｅｃｏｎ⁃ｆｉｇｕｒａｂｌｅＬｉｎｅａｒ⁃ｔｏ⁃ＣｉｒｃｕｌａｒＰｏｌａｒｉｚｅｒ／ＲｅｆｌｅｃｔｏｒＢａｓｅｄｏｎＰＩＮＤｉｏｄｅｓ［Ｊ］．ＰｈｙｓｉｃａＳｃｒｉｐｔａ，２０２１，９６（１２）：１２５８４６．［１３］ＳＩＮＧＨＨ，ＭＩＴＴＡＬＮ，ＧＵＰＴＡＡ，ｅｔａｌ．ＭｅｔａｍａｔｅｒｉａｌＩｎｔｅ⁃ｇｒａｔｅｄＦｏｌｄｅｄＤｉｐｏｌｅＡｎｔｅｎｎａｗｉｔｈＬｏｗＳＡＲｆｏｒ４Ｇ，５ＧａｎｄＮＢ⁃ＩｏＴＡｐｐｌｉｃａｔｉｏｎｓ［Ｊ］．Ｅｌｅｃｔｒｏｎｉｃｓ，２０２１，１０（２１）：２６１２．［１４］盛丽丽，曹卫平，梅立荣，等．基于数字超表面的低剖面波束控制天线［Ｊ］．电波科学学报，２０２１，３６（６）：９３８－９４６．［１５］张磊，陈晓晴，郑熠宁，等．电磁超表面与信息超表面［Ｊ］．电波科学学报，２０２１，３６（６）：８１７－８２８．［１６］ＷＡＮＺＷ，ＧＡＯＺ，ＧＡＯＦＦ，ｅｔａｌ．ＴｅｒａｈｅｒｔｚＭａｓｓｉｖｅＭＩＭＯｗｉｔｈＨｏｌｏｇｒａｐｈｉｃＲｅｃｏｎｆｉｇｕｒａｂｌｅＩｎｔｅｌｌｉｇｅｎｔＳｕｒｆａｃｅｓ［Ｊ］．ＩＥＥＥＴｒａｎｓａｃｔｉｏｎｓｏｎＣｏｍｍｕｎｉｃａｔｉｏｎｓ，２０２１，６９（７）：４７３２－４７５０．［１７］ＺＨＡＮＧＹ，ＴＡＮＹＷ，ＳＴＯＲＭＥＲＨＬ，ｅｔａｌ．ＥｘｐｅｒｉｍｅｎｔａｌＯｂｓｅｒｖａｔｉｏｎｏｆｔｈｅＱｕａｎｔｕｍＨａｌｌＥｆｆｅｃｔａｎｄＢｅｒｒｙｓＰｈａｓｅｉｎＧｒａｐｈｅｎｅ［Ｊ］．Ｎａｔｕｒｅ，２００５，４３８（７０６５）：２０１－２０４．［１８］ＭＡＹＯＲＯＶＡＳ，ＧＯＲＢＡＣＨＥＶＲＶ，ＭＯＲＯＺＯＶＳＶ，ｅｔａｌ．ＭｉｃｒｏｍｅｔｅｒＳｃａｌｅＢａｌｌｉｓｔｉｃＴｒａｎｓｐｏｒｔｉｎＥｎｃａｐｓｕｌａｔｅｄＧｒａｐｈｅｎｅａｔＲｏｏｍＴｅｍｐｅｒａｔｕｒｅ［Ｊ］．ＮａｎｏＬｅｔｔｅｒｓ，２０１１，１１（６）：２３９６－２３９９．［１９］ＫＯＰＰＥＮＳＦ，ＣＨＡＮＧＤＥ，ＡＢＡＪＯＦＤ．ＧｒａｐｈｅｎｅＰｌａｓ⁃ｍｏｎｉｃｓ：ＡＰｌａｔｆｏｒｍｆｏｒＳｔｒｏｎｇＬｉｇｈｔＭａｔｔｅｒＩｎｔｅｒａｃｔｉｏｎ［Ｊ］．ＮａｎｏＬｅｔｔｅｒｓ，２０１１，１１（８）：３３７０－３３７７．［２０］ＬＩＺ，ＨＵＡＮＧＴ，ＬＩＵＧＢ，ｅｔａｌ．ＡＴｕｎａｂｌｅＵｌｔｒａ⁃Ｂｒｏａｄ⁃ｂａｎｄＬｉｎｅａｒ⁃ｔｏ⁃ＣｉｒｃｕｌａｒＰｏｌａｒｉｚａｔｉｏｎＣｏｎｖｅｒｔｅｒＣｏｎｔａｉｎｉｎｇｔｈｅＧｒａｐｈｅｎｅ［Ｊ］．ＯｐｔｉｃｓＣｏｍｍｕｎｉｃａｔｉｏｎｓ，２０１９，４３６：７－１３．［２１］ＧＨＯＳＨＳＫ，ＤＡＳＳ，ＢＨＡＴＴＡＣＨＡＲＹＹＡＳ．Ｔｒａｎｓｍｉｔｔｉｖｅ⁃ｔｙｐｅＴｒｉｐｌｅ⁃ｂａｎｄＬｉｎｅａｒｔｏＣｉｒｃｕｌａｒＰｏｌａｒｉｚａ⁃ｔｉｏｎＣｏｎｖｅｒｓｉｏｎｉｎＴＨｚＲｅｇｉｏｎＵｓｉｎｇＧｒａｐｈｅｎｅ⁃ｂａｓｅｄＭｅｔａｓｕｒｆａｃｅ［Ｊ］．ＯｐｔｉｃｓＣｏｍｍｕｎｉｃａｔｉｏｎｓ，２０２０，４８０：１２６４８０．［２２］ＧＵＡＮＳ，ＣＨＥＮＧＪＲ，ＣＨＥＮＴ，ｅｔａｌ．ＷｉｄｅｌｙＴｕｎａｂｌｅＰｏｌａｒｉｚａｔｉｏｎＣｏｎｖｅｒｓｉｏｎｉｎＬｏｗ⁃ｄｏｐｅｄＧｒａｐｈｅｎｅ⁃ｄｉｅｌｅｃｔｒｉｃＭｅｔａｓｕｒｆａｃｅｓＢａｓｅｄｏｎＰｈａｓｅＣｏｍｐｅｎｓａｔｉｏｎ［Ｊ］．ＯｐｔｉｃｓＬｅｔｔｅｒｓ，２０２０，４５（７）：８－１２．［２３］ＨＡＮＳＯＮ，ＧＥＯＲＧＥＷ．ＤｙａｄｉｃＧｒｅｅｎｓＦｕｎｃｔｉｏｎｓａｎｄＧｕｉｄｅｄＳｕｒｆａｃｅＷａｖｅｓｆｏｒａＳｕｒｆａｃｅＣｏｎｄｕｃｔｉｖｉｔｙＭｏｄｅｌｏｆＧｒａｐｈｅｎｅ［Ｊ］．ＪｏｕｒｎａｌｏｆＡｐｐｌｉｅｄＰｈｙｓｉｃｓ，２００８，１０３（６）：１９９１２．［２４］ＺＨＡＮＧＹ，ＦＥＮＧＹ，ＺＨＡＯＪ．Ｇｒａｐｈｅｎｅ⁃ｅｎａｂｌｅｄＴｕｎａｂｌｅＭｕｌｔｉｆｕｎｃｔｉｏｎａｌＭｅｔａｍａｔｅｒｉａｌｆｏｒＤｙｎａｍｉｃａｌＰｏｌａｒｉｚａｔｉｏｎＭａｎｉｐｕｌａｔｉｏｎｏｆＢｒｏａｄｂａｎｄＴｅｒａｈｅｒｔｚＷａｖｅ［Ｊ］．Ｃａｒｂｏｎ，２０２０，１６３：２４４－２５２．［２５］ＭＵＲＴＡＺＡＭ，ＲＡＳＨＩＤＡ，ＴＡＨＩＲＦＡ．ＡＨｉｇｈｌｙＥｆｆｉｃｉｅｎｔＬｏｗ⁃ｃｏｓｔＲｅｆｌｅｃｔｉｖｅＡｎｉｓｏｔｒｏｐｉｃＭｅｔａｓｕｒｆａｃｅｆｏｒＬｉｎｅａｒＴｏＬｉｎｅａｒｌｙＣｒｏｓｓ⁃ａｎｄＣｉｒｃｕｌａｒ⁃ｐｏｌａｒｉｚａｔｉｏｎＣｏｎ⁃ｖｅｒｓｉｏｎ［Ｊ］．ＭｉｃｒｏｗａｖｅａｎｄＯｐｔｉｃａｌＴｅｃｈｎｏｌｏｇｙＬｅｔｔｅｒｓ，２０２０，６３（５）：１３４６－１３５３．［２６］唐小燕，柯友煌，井绪峰，等．基于透射型几何相位编码超表面的太赫兹波束自由操控［Ｊ］．光子学报，２０２１，５０（１）：１５０－１６３．［２７］ＬＩＹ，ＬＩＨ，ＷＡＮＧＹ，ｅｔａｌ．ＡＮｏｖｅｌＳｗｉｔｃｈａｂｌｅＡｂｓｏｒｂｅｒ／ＬｉｎｅａｒＣｏｎｖｅｒｔｅｒＢａｓｅｄｏｎＡｃｔｉｖｅＭｅｔａｓｕｒｆａｃｅａｎｄＩｔｓＡｐ⁃ｐｌｉｃａｔｉｏｎ［Ｊ］．ＩＥＥＥＴｒａｎｓａｃｔｉｏｎｓｏｎＡｎｔｅｎｎａｓａｎｄＰｒｏｐａ⁃ｇａｔｉｏｎ，２０２０，６８（１１）：７６８８－７６９３．［２８］吕欢欢．基于人工电磁表面的天线研究及其可重构设计［Ｄ］．西安：西安电子科技大学，２０２０．［２９］ＣＨＥＮＹ，ＹＩＬ，ＧＵＯＪ，ｅｔａｌ．ＷｉｄｅｂａｎｄＴｕｎａｂｌｅＭｉｄ⁃ｉｎ⁃ｆｒａｒｅｄＣｒｏｓｓＰｏｌａｒｉｚａｔｉｏｎＣｏｎｖｅｒｔｅｒＵｓｉｎｇＲｅｃｔａｎｇｌｅ⁃ｓｈａｐｅＰｅｒｆｏｒａｔｅｄＧｒａｐｈｅｎｅ［Ｊ］．ＯｐｔｉｃｓＥｘｐｒｅｓｓ，２０１６，２４（１５）：１６９１３．［３０］崔铁军．电磁超材料从等效媒质到现场可编程系统［Ｊ］．中国科学：信息科学，２０２０，５０（１０）：１４２７－１４６１．作者简介：㊀㊀司黎明㊀工学博士，副教授㊂主要研究方向：电磁场与微波技术㊂㊀㊀汤鹏程㊀博士研究生㊂主要研究方向：电磁场与微波技术㊂㊀㊀吕㊀昕㊀工学博士，教授㊂主要研究方向：电磁场与微波技术㊂。

基于石墨烯超表面的效率可调太赫兹聚焦透镜

DOI: 10.12086/oee.2021.200319基于石墨烯超表面的效率可调太赫兹聚焦透镜王俊瑶，樊俊鹏，舒好，刘畅，程用志*武汉科技大学信息科学与工程学院，湖北武汉 430081摘要：本文提出了一种基于石墨烯超表面的效率可调太赫兹聚焦透镜。

该超表面单元结构由两层对称的圆形镂空石墨烯和中间介质层组成，其中镂空圆形中间由长方形石墨烯片连接。

该结构可实现偏振转换，入射到超表面的圆偏振波将以其正交的形式出射，如左旋圆到右旋圆偏振转换。

利用几何相位原理，通过旋转长方形条的方向，透射波会携带额外的附加相位并能满足2π范围内覆盖。

合适地排列石墨烯超表面的单元结构，以实现太赫兹聚焦透镜。

仿真结果表明：通过改变石墨烯的费米能级，可以对超表面圆偏振转换幅度进行调节，进而超透镜的聚焦效率也可以动态调节。

因此，这种基于石墨烯超表面的效率可调聚焦透镜不用改变单元结构的尺寸，只需通过改变费米能级便可实现，可以广泛地应用到能量收集、成像等太赫兹应用领域。

关键词：超表面；聚焦透镜；石墨烯；太赫兹中图分类号：TH74；TQ127.11 文献标志码：A王俊瑶，樊俊鹏，舒好，等. 基于石墨烯超表面的效率可调太赫兹聚焦透镜[J]. 光电工程，2021，48(4): 200319Wang J Y , Fan J P , Shu H, et al. Efficiency-tunable terahertz focusing lens based on graphene metasurface[J]. Opto-Electron Eng , 2021, 48(4): 200319Efficiency-tunable terahertz focusing lens based on graphene metasurfaceWang Junyao, Fan Junpeng, Shu Hao, Liu Chang, Cheng Yongzhi *School of Information Science and Engineering, Wuhan University of Science and Technology, Wuhan, Hubei 430081, China Abstract: This paper proposes an efficiency-tunable terahertz focusing lens based on the graphene metasurface. The unit cell is composed of two symmetrical circular graphene hollows and an intermediate dielectric layer, wherein the hollow circular middle is connected by a rectangular graphene sheet. This structure can realize polarization conversion, for example, when an incidence with left-hand circular polarization emitted on the metasurface the po-larization of the transmitted light is right-hand circular polarization. According to the principle of geometric phase, by rotating the direction of the rectangular bar, the transmitted wave will carry an additional phase and can cover the range of 2π. An THz focusing lens can be realized by properly arranging the unit structure of the graphene metasurface. The simulation results show that the conversion amplitude of circular polarized light can be adjusted by changing the Fermi level of graphene, and the focusing efficiency of the metalens can also be dynamically adjusted.LCPRCP(cross-polarization)xy zV g——————————————————收稿日期：2020-08-27；收到修改稿日期：2020-10-26基金项目：湖北省教育厅科技研究计划重点项目(D2*******)；武汉科技大学研究生创新基金项目(JCX201959)；大学生创新基金项目资助课题(20ZA083)作者简介：王俊瑶(2000-)，女，主要从事电子科学与技术专业。

基于石墨烯超材料的宽频带可调太赫兹吸波体

第19卷第6期太赫兹科学与电子信息学报Vo1.19，No.62021年12月 Journal of Terahertz Science and Electronic Information Technology Dec.，2021文章编号：2095-4980(2021)06-0973-06基于石墨烯超材料的宽频带可调太赫兹吸波体胡丹1，付麦霞2，朱巧芬3(1.安阳师范学院物理与电气工程学院，河南安阳 455000；2.河南工业大学信息科学与工程学院，河南郑州 450001；3.河北工程大学数理科学与工程学院，河北邯郸 056038)摘要：基于二维材料石墨烯，设计了一款宽频带可调谐超材料太赫兹吸波体。

该吸波体由三层结构组成，顶层为石墨烯超材料，中间层为二氧化硅，底层为金属薄膜。

仿真结果表明，当石墨烯的费米能级为0.7eV时，该吸波体在1.11~2.61THz频率范围内吸收率超过90%，相对吸收带宽为80.6%。

当石墨烯的费米能级从0eV增大到0.7eV时，该吸波体器件的峰值吸收率可以从20.32%增大到98.56%。

此外，该吸波体器件还具有极化不敏感和广角吸收的特性。

因此，它在太赫兹波段的热成像、热探测、隐身技术等领域具有潜在的应用价值。

关键词：超材料；太赫兹；吸波体；石墨烯中图分类号：TN29文献标志码：A doi：10.11805/TKYDA2021248Tunable broadband terahertz absorber based on graphene metamaterialHU Dan1，FU Maixia2，ZHU Qiaofen3(1.School of Physics and Electrical Engineering，Anyang Normal University，Anyang Henan 455000，China；. All Rights Reserved.2.College of Information Science and Engineering，Henan University of Technology，Zhengzhou Henan 450001，China)3.School of Mathematics and Physics Science and Engineering，Hebei University of Engineering，Handan Hebei 056038，China)Abstract：A tunable broadband terahertz absorber based on graphene metamaterial is proposed and numerically demonstrated. The absorber consists of three layers: the upper is the graphene metamateriallayer, the middle is the SiO2layer, and the bottom is the metallic layer. Simulation results demonstratethat the proposed absorber achieves over 90% absorption in 1.11- 2.61THz with a relative bandwidth of80.6%when Fermi level c=0.7eV. The peak absorption rate of the proposed absorber can be tuned from20.32%to 98.56%by changing the Fermi energy of graphene from 0eV to 0.7eV. Additionally, theproposed absorber is insensitive to polarization and has high absorbance to wide incidence angles. Suchdesign may have some potential applications in thermal imaging, thermal detecting, and stealth technique.Keywords：metamaterial；terahertz；absorber；graphene超材料吸波体具有厚度薄、质量轻、吸收能力强、高度集成等优点，并且可以“量需定制”。

Visible-Light-DrivenPhotocatalyticHydrogenProductionofCdS-Cluster-DecoratedGrapheneNanosheets

Published:June 03,2011ARTICLE/JACSHighly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene NanosheetsQin Li,†,‡Beidou Guo,†Jiaguo Yu,*,‡Jingrun Ran,‡Baohong Zhang,†Huijuan Yan,§and Jian Ru Gong*,††National Center for Nanoscience and Technology,11Zhongguancun Beiyitiao,Beijing 100190,People ’s Republic of China ‡State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Technology,Wuhan 430070,People ’s Republic of China §Institute of Chemistry,The Chinese Academy of Science,Beijing 100080,People ’s Republic of China1.INTRODUCTIONThe increasingly serious energy crisis and the environmental contamination caused by the burning of fossil fuels have led to an aggressive search for renewable and environmental-friendly alternative energy recourses.1Hydrogen energy has been recog-nized as a potentially signi ﬁcant alternative form of storable and clean energy for the future.Since the ﬁrst report on photocata-lytic splitting of water on TiO 2electrodes was published in 1972by Honda and Fujishima,photocatalysis has demonstrated wide-ranging potential applications in areas such as converting solar energy,recycling polluted water or air,and so on.2À9The research on the photocatalytic splitting of water to produce hydrogen,mimicking natural photosynthesis by converting solar energy into chemical energy,has been carried on extensively.10À12Recently,researchers have been focusing on the development of visible-light-responsive photocatalysts,be-cause the ultraviolet (UV)light only accounts for about 4%of the solar radiation energy,while the visible light contributes to about 43%.13À16As compared to their wide bandgap counterparts,chalcogen-ide nanomaterials,particularly CdS particles,are attractive photocatalytic materials for the conversion of solar energy into chemical energy under visible-light irradiation.Speci ﬁcally,these materials have a conduction band edge su ﬃciently more negative than the reduction potential of protons and a relatively narrow bandgap,which can e ﬃciently absorb visible light.17,18However,there are several issues that still limit the H 2-production rate onpure CdS particles.For example,the CdS particles tend to aggregate,forming larger particles,which results in a reduced surface area and a higher recombination rate of photoinduced electron Àhole pairs.To solve these problems,many approaches have been proposed to enhance the photocatalytic activity of CdS particles,including the preparation of quantum-sized CdS,19deposition of noble metals,20preparation of heterogeneous semiconductors,14and incorporation of semiconductor particles in the interlayer region of layered compounds.21For example,Bard et al.22introduced CdS particles into colloidal suspensions of clay;Sato et al.23and Hirai et al.24incorporated CdS and/or ZnS particles into the interlayer of hydrotalcite and mesoporous silica,respectively.The layered structure of such a supporting matrix can e ﬃciently suppress the growth of semiconductor particles as well as facilitate the transfer of the photogenerated electrons to the surface of photocatalysts.Furthermore,the recom-bination between the photoinduced charge carriers can be e ﬀec-tively suppressed,leading to the high e ﬃciency of H 2production.There has been an explosion of interest in graphene since its discovery by Geim et al.in 2004due to its potential applications in the physical,chemical,biological,photoelectric,and catalytic ﬁelds.25For example,Jiang et al.26synthesized graphene ÀCdS (G ÀCdS)nanocomposites by a re ﬂux approach using thio salts as the sulphide source and hydrazine hydrate as the reducingReceived:March 21,2011ABSTRACT:The production of clean and renewable hydrogen through water splitting using photocatalysts has received much attention due to the increasing global energy crises.In this study,a high e ﬃciency of the photocatalytic H 2production was achieved using graphene nanosheets decorated with CdS clus-ters as visible-light-driven photocatalysts.The materials were prepared by a solvothermal method in which graphene oxide (GO)served as the support and cadmium acetate (Cd(Ac)2)as the CdS precursor.These nanosized composites reach a high H 2-production rate of 1.12mmol h À1(about 4.87times higherthan that of pure CdS nanoparticles)at graphene content of 1.0wt %and Pt 0.5wt %under visible-light irradiation and an apparent quantum e ﬃciency (QE)of 22.5%at wavelength of 420nm.This high photocatalytic H 2-production activity is attributed predominantly to the presence of graphene,which serves as an electron collector and transporter to e ﬃciently lengthen the lifetime of the photogenerated charge carriers from CdS nanoparticles.This work highlights the potential application of graphene-based materials in the ﬁeld of energyconversion.agent;Nethravathi et al.27prepared GÀCdS/ZnS composites using H2S gas as the sulphide source as well as reducing agent; Cao et al.28utilized a solvothermal method to synthesize a GÀCdS nanocomposite material with good structural and optoelectronic properties,using dimethyl sulfoxide(DMSO) instead of H2S.The solvothermal method not only avoids the use of toxic hydrazine hydrate,but also allows control over the degree of reduction of GO.In this way,the residual oxygen-containing hydrophilic groups on the graphene may allow the composite to be dispersed in water to a certain extent,which is required for the photocatalytic reaction.29To the best of our knowledge,no previous work regarding the application of GÀCdS on the photocatalytic H2production has been reported.However,the two-dimensional(2D)platform structure of graphene makes it an excellent supporting matrix for photocatalyst particles,similar to the role of layer-structured matrices played in improving the eﬃciency of the photocatalysts as mentioned above.30Moreover,because of the excellent electronic conductivity of graphene imparted by its2D planar π-conjugation structure,it can eﬀectively inhibit the recombina-tion of the electronÀhole pairs in the GÀCdS nanocomposites.31,32 In addition,an appropriate amount of graphene may darken the composites and thus enhance the absorption of visible light.In this work,the inﬂuence of graphene on the properties of the CdS clusters was systematically investigated,and high eﬃciency of the visible-light-driven photocatalytic H2production was achieved using the CdS-cluster-decorated graphene nanosheets as the photocatalyst.Furthermore,a mechanism for photocatalytic re-action in the grapheneÀCdS system is proposed.2.EXPERIMENTS2.1.Sample Preparation.The CdS-cluster-decorated graphene nanosheets(GÀCdS)were prepared by a solvothermal method.28All of the reagents were of analytical grade and were used without further purification.Deionized(DI)water was used in all experiments.Gra-phene oxide(GO)was synthesized from natural graphite powder (>99.8%,Alfa Aesar)by a modified Hummers’method.33In a typical synthesis of the composite,a varying amount of the prepared GO and1.6 mmol of Cd(Ac)232H2O(∼98.5%,Aladdin)were dispersed in160mL of DMSO.The weight ratios of GO to Cd(Ac)232H2O were0,0.5%, 1.0%,2.5%,5.0%,and40%,and the obtained samples were labeled as GC0,GC0.5,GC1.0,GC2.5,GC5.0,and GC40,respectively.Next,the homogeneous solution was transferred into a200mL Teflon-lined autoclave and held at180°C for12h after vigorous stirring and sonication.After that,the precipitates from the mixture were allowed to cool to room temperature and collected by centrifugation,and then rinsed with acetone and ethanol several times to remove the residue of DMSO.The final product was dried in an oven at60°C for12h.The bare graphene sample without any CdS clusters was prepared under the same experimental conditions for the purpose of comparison and was labeled as G.2.2.Characterization.Powder X-ray diffraction(XRD)patterns were obtained on a D/MAX-2500diffractometer(Rigaku,Japan)using Cu K R radiation source(λ=1.54056Å)at a scan rate of5°minÀ1to determine the crystal phase of the obtained samples.The accelerating voltage and the applied current were50kV and300mA,respectively. The average crystallite sizes were calculated using the Scherrer formula (d=0.9λ/B cosθ,where d,λ,B,andθare crystallite size,Cu K R wavelength,full width at half-maximum intensity(fwhm)in radians,and Bragg’s diffraction angle,respectively).Scanning electron microscopy (SEM)images were collected on an S-4800field emission SEM (FESEM,Hitachi,Japan).Transmission electron microscopy(TEM)images,high-resolution transmission electron microscopy(HRTEM) images,and selected area electron diffraction(SAED)patterns were collected on an F20S-TWIN electron microscope(Tecnai G2, FEI Co.),using a200kV accelerating voltage.The BrunauerÀEmmettÀTeller(BET)specific surface area(S BET)of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP2020 nitrogen adsorption apparatus(U.S.).All of the prepared samples were degassed at180°C prior to nitrogen adsorption measurements.The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure(P/P0)range of0.05À0.3. The desorption isotherm was used to determine the pore size distribu-tion using the BarretÀJoynerÀHalender(BJH)method,assuming a cylindrical pore modal.UVÀvis diffused reflectance spectra of the samples were obtained from a UVÀvis spectrophotometer(UV2550, Shimadzu,Japan).BaSO4was used as a reflectance standard.X-ray photoelectron spectroscopy(XPS)data were obtained by an ESCALa-b220i-XL electron spectrometer from VG Scientific using300W Al K R radiation.The base pressure was about3Â10À9mbar.The binding energies were referenced to the C1s line at284.8eV from adventitious carbon.Atomic force microscopy(AFM)images were obtained by a Dimension3100AFM,operating in tapping mode with a scan rate of 1.20Hz and a resolution of256Â256.An n-doped silicon tip with1À10Ωcm phosphorus(Veeco,MPP-11100-140)was used as the probe. Fourier transform infrared spectra(FTIR)of the samples were recorded between500and2000cmÀ1on an IRAffinity-1FTIR spectrometer.2.3.Photocatalytic Hydrogen Production.The photocatalytic hydrogen production experiments were performed in a100mL Pyrex round-bottom flask,the openings of which were sealed with a silicone rubber septum,at ambient temperature and atmospheric pressure.A 350W xenon arc lamp with a UV-cutoff filter(g420nm)was used as a visible light source to trigger the photocatalytic reaction and was positioned20cm away from the reactor.The focused intensity on the flask was ca.180mW cmÀ2,which was measured by an FZ-A visible-light radiometer(made in the photoelectric instrument factory of Beijing Normal University,China)over the wavelength in the range of 400À1000nm.34In a typical photocatalytic experiment,20mg of the prepared GÀCdS photocatalyst was dispersed with constant stirring in an80mL mixed solution of lactic acid(8mL)and water(72mL).A certain amount of H2PtCl636H2O aqueous solution was dripped into the system to load 0.5wt%Pt onto the surface of the photocatalyst by a photochemical reduction deposition method.Prior to irradiation,the system was bubbled with nitrogen for30min to remove the dissolved oxygen. During the whole reduction process,agitation of the solution ensured uniform irradiation of the GÀCdS suspension.A0.4mL sample of the generated gas was collected intermittently through the septum,and hydrogen content was analyzed by gas chromatograph(GC-14C, Shimadzu,Japan,TCD,nitrogen as a carrier gas and5Åmolecular sieve column).All glassware was rigorously cleaned and carefully rinsed with distilled water prior to use.The apparent quantum eﬃciency(QE)was measured under the same photocatalytic reaction condition except that four420nm-LEDs(3W) (Shenzhen LAMPLIC Science Co.Ltd.,China)were used as light sources to trigger the photocatalytic reaction,instead of the xenon arc lamp.The LEDs were positioned1cm away from the reactor in four diﬀerent directions, and the focused intensity on theﬂask for each of them was ca.6.0mW cmÀ2 over an area of1cm2.The QE was calculated according to eq1:35QE½% ¼number of reacted electronsnumber of incident photonsÂ100¼number of evolved H2moleculesÂ2number of incident photonsÂ100ð1Þ3.RESULTS AND DISCUSSION3.1.Phase Structures and Morphology.XRD patterns wererecorded for the dried G ÀCdS powder to confirm the crystal-lographic phase of CdS in the composite and investigate the influence of graphene on the crystallinity of CdS nanoparticles.Figure 1shows the XRD patterns of G ÀCdS nanocomposites synthesized with different contents of graphene as compared to that of the pure CdS (i.e.,GC0).The peaks at 26.5°,44.0°,and 52.1°correspond to the diffractions of the (111),(220),and (311)planes of cubic CdS (JCPDS 80-0019),respectively.The diffraction peaks are broad because the crystallite sizes of CdS nanoparticles in the samples are relatively small.In general,the solubility product constant (K sp )for CdS particles is quite small,leading to fast nucleation and agglomeration of CdS nanocrystals.13However,DMSO can regulate the nucleation rate of CdS particles by slowly releasing S 2Àions into solution,resulting in a much smaller crystallite size.No characteristic diffraction peaks for carbon species are observed in the patterns because of the low amount and relatively low diffraction intensity of graphene.The XRD patterns also imply that graphene may enhance the crystallinity of CdS particles.As shown in Figure 1,pure CdS particles (GC0,yellow line)have poor crystallinity,perhaps because the reactor conditions are not ideal for their nucleation.After introducing 0.5%graphene,the XRD peaks of sample GC0.5(brown line)become stronger and narrower due to the improved crystallinity of CdS particles.As the graphene content is increased,the intensity of XRD peaks is correspond-ingly enhanced.To further highlight this effect,the average crystallite sizes of different samples were calculated using the Scherrer formula for the (111)facet diffraction peak.As shown in Table 1,the average crystallite size of CdS particles increases from 2.6to 3.1nm.Thus,it can be deduced that the layer structure of graphene supplies a platform on which the CdS nanoparticles can nucleate,and thus graphene can promote the crystallization of CdS nanoparticles to a certain extent.Furthermore,the morphologies of samples GC0and GC1.0were analyzed by SEM to directly observe the structure of the graphene nanosheets decorated with CdS clusters,and tospeci ﬁcally investigate the in ﬂuence of graphene on the mor-phology of the CdS clusters.The SEM micrograph in Figure 2a shows a signi ﬁcant aggregation of the CdS nanoparticles in sample GC0and particle diameters of approximate 100nm.However,Figure 2b shows that much smaller CdS clusters spread uniformly and tightly on the graphene sheets in sample GC1.0,indicating that graphene may interact with CdS nanoparticles and inhibit their aggregation.As has been reported previously,nanoparticles may interact with graphene sheets through physi-sorption,electrostatic binding,or charge transfer interaction,36and the exact mechanism is still under investigation.The TEM image of GC1.0(Figure 3a)shows that many small CdS particles are present on the graphene sheet,which has a characteristic wrinkle on the edge.The result further con ﬁrms the combination of graphene nanosheet and CdS clusters,which is consistent with the SEM image (Figure 2b).The HRTEM image (Figure 3b)shows that the size of the CdS clusters in GC1.0is about 3nm,which is in agreement with the value calculated by the Scherrer formula (Table 1).The lattice fringesTable 1.E ﬀects of Graphene Content on Physicochemical Properties and Quantum E ﬃciency (QE)of the Graphene ÀCdS Samplessamples graphene content(wt %)crystallite size a(nm)S BET (m 2g À1)QE (%)GC00 2.635.5 4.6GC0.50.5 2.640.77.7GC1.0 1.0 2.748.222.5GC2.5 2.5 2.758.211.1GC5.0 5.0 3.054.2 4.6GC40403.196.80.6aAverage crystallite size is determined by the broadening of the CdS (111)facetdi ﬀraction peak using the Scherrer formula.Figure 1.XRD patterns of sample GC x solid powders (x=0,0.5,1.0,2.5,5.0,and 40).Figure 2.SEM images of(a)sample GC0and (b)sample GC1.0.Figure 3.(a)TEM and (b)HRTEM images of sample GC1.0,with the inset of (b)showing the SAED pattern of graphene sheet decorated with CdS clusters.of individual CdS clusters with d spacing of ca.0.336,0.206,and 0.177nm can be assigned to the (111),(220),and (311)lattice planes of the cubic CdS,respectively.The selected area electron di ﬀraction (SAED)pattern (inset in Figure 3b)indicates that these nanoparticles are polycrystalline.The three inside di ﬀrac-tion rings correspond to the (111),(220),(311)planes of the cubic CdS,which is fully consistent with the XRD results (Figure 1).In addition,the well-de ﬁned di ﬀraction ring with six spots,ﬁve of which are indicated by the arrows in the SAED pattern,implies that thin,ﬂat graphene ﬁlms were obtained via the reduction of GO.For comparison,the morphology of GO was also character-ized by SEM and AFM.Unlike the nanocomposites,GO nanosheets without any CdS clusters have a crumpled shape in the SEM image (Figure 4a),suggesting that CdS clusters can prohibit the crumpling and agglomeration of graphene na-nosheets during the solvothermal reduction process.The AFM image (Figure 4b)shows a 2D GO nanosheet with wrinkle-like features,and the apparent thickness is ca.0.754nm,which is comparable to the literature data (0.737nm)for the single-layer GO nanosheet as reported before.373.2.BET Surface Areas and Pore Size Distributions.The effect of graphene on the BET surface area and pore structure of the prepared samples was investigated using adsorption Àdesorp-tion measurements.As shown in Table 1,the BET surface area (S BET )of samples gradually increases with increasing graphene content,from 35.5to 96.8m 2g À1.It should be noted that the specific surface area (m 2g À1)is expressed per gram of the samples,which contain some amount of carbon (graphene)with a low density.The planar density of graphene is 0.77mg cm À2,and the density of CdS is4.82g cm À3.Consequently,the average densities of samples decrease with the increasing graphene,resulting in the increase of the S BET .A greater specific surface area of photocatalysts can supply more surface active sites and make charge carriers transport easier,leading to an enhancement of the photocatalytic performance.38Thus,graphene may play a role in enhancing the photocatalytic activity.Figure 5shows the nitrogen adsorption Àdesorption iso-therms and the corresponding curves of the pore size distribution (inset)for samples GC0,GC1.0,GC5.0,and GC40.According to the Brunauer ÀDeming ÀDeming ÀTeller (BDDT)classi ﬁcation,the majority of physisorption isotherms can be grouped into six types.39Typically,pure CdS (sample GC0)has an isotherm of type II,indicating the presence of large macropores,while samples GC1.0,GC5.0,and GC40have isotherms of type IV,suggesting the presence of mesopores.39The shape of the hysteresis loops is of type H3,associated with slit-like pores formed by the aggregations of the plate-like particles.In other words,it appears that the prepared nanocomposite is composed of sheet-like graphene decorated with CdS clusters.The results are further con ﬁrmed by the corresponding pore size distribu-tion.As shown in the inset of Figure 5,sample GC0has macropores with a peak pore diameter of around 100nm.When graphene is introduced,mesopores begin to appear in samples with a typical pore diameter of around 4nm,and the amount of macropores decreases.3.3.UV ÀVis Diffuse Reflection Spectra.A comparison of the UV Àvis diffuse reflectance spectra of samples GC x (x =0,0.5,1.0,2.5,5.0,and 40)is displayed in Figure 6.There is an enhanced absorbance in the visible-light region (>500nm)with increasing graphene content.This is also observed as a color change of the samples,which become darker,that is,from pale yellow to olive,when a low amount of black graphene was introduced into the pure CdS nanoparticles.The results show that the addition of graphene increases the absorbance of visible light.When the graphene content reaches 40%(sample GC40),the absorbance is significantly higher than that of the other samples used inthis study.Because of the increased absorbance,aFigure 4.(a)SEM and (b)AFM images of GO sheets,with the inset of (b)showing that the thicknessof the GO fragment is ca.0.754nm.Figure 5.Nitrogen adsorption Àdesorption isotherms and correspond-ing pore size distribution curves (inset)of samples GC x solid powders (x =0,1.0,5.0,and 40).Figure 6.UV Àvis di ﬀuse re ﬂectance spectra of samples GC x solid powders (x =0,0.5,1.0,2.5,5.0,and 40).more efficient utilization of the solar energy can be obtained.Therefore,we can infer that the introduction of graphene in CdS particles is effective for the visible-light response of the composite.3.4.XPS and FTIR Spectra.The hydrophilic groups on the surface of graphene,such as hydroxyl and carboxyl groups,can enhance the dispersion of graphene in water,providing the prerequisite for the H 2generation occurring in aqueous solution.To investigate the degree of reduction of GO in the solvothermal reduction process,high-resolution XPS spectra of C1s were collected from samples GO and GC1.0(Figure 7).The XPS spectrum of C1s from GO (Figure 7a,solid line)can be deconvoluted into four smaller peaks (dashed lines),which are ascribed to the following functional groups:sp 2bonded carbon (C ÀC,284.8eV),epoxy/hydroxyls (C ÀO,286.9eV),carbonyls (C d O,287.8eV),and carboxyl (O ÀC d O,288.9eV),40,41indicating the high percentage of oxygen-containing functional groups.In comparison,in the XPS spectrum of C1s from GC1.0(Figure 7b,solid line),the peak for C d O almost vanishes,and the peaks for C ÀO and O ÀC d O (dashed lines)still exist but with much lower intensities than those in GO,indicating the partial removal of the oxygen-containing functional groups.Furthermore,the degree of reduction of GO can be quantified by calculating the relative content of carbon in the samples.Briefly,GO has 48%graphitic carbon and 52%oxidized carbon,while 68%graphitic carbon and 32%oxidized carbon for GC1.0,respectively,showing the loss of oxygen-containing functional groups and the partial reduction of GO by the solvothermal reduction process.Thus,the graphene sheets in the composite can be dispersed in the aqueous solution to a certain extent.Further evidence for the existence of the hydrophilic groups on the surface of graphene comes from FTIR spectra.In Figure 8,the characteristic bands of GO are observed at 972cm À1(epoxy stretching),1057cm À1(alkoxy C ÀO stretching),1224cm À1(phenolic C ÀOH stretching),1402cm À1(carboxyl O ÀH stretching),and 1724cm À1(C d O stretching vibrations of carboxyl or carbonyl groups).42The peak at 1120cm À1is ascribed to C ÀO stretching vibrations of CO 2,and the broad absorption at 1624cm À1is related to H ÀO ÀH bending band of the adsorbed H 2O molecules or the in-plane vibrations of sp 2-hybridized C ÀC bonding.33As compared to the peaks of the functional groups of GO,sample GC1.0has a similar spectrum but with much lower absorption intensity,especially for the peaks at 1224,1402,and 1724cm À1,which are all assigned to the oxygen-containing functional groups.This result indicates the partial reduction of GO and is in good agreement with the XPS results.3.5.Photocatalytic Activity and Tentative Mechanism of Photocatalytic Reaction.Recently,Zhang et al.43synthesized a graphene ÀTiO 2nanocomposite and applied it as thephotocatalyst in hydrogen production from water splitting under irradiation of UV Àvis light,obtaining a maximum H 2-produc-tion rate of 8.6μmol h À1.The results demonstrate that graphene is a very promising candidate for developing photocatalysts with high performance.However,because the bandgap of TiO 2is wide and visible light cannot be utilized effectively,the efficiency of H 2production is low.The applicability of chalcogenide nano-materials has also been widely explored on hydrogen production.For example,Xu et al.44prepared (Zn 0.95Cu 0.05)1Àx Cd x S solid solutions,which gave an H 2-production rate of 1.09mmol h À1when loaded with 0.75wt %Pt;Bao et al.13synthesized nanoporous CdS nanostructures to increase the Pt loading content,and the maximum rate of H 2production was 4.1mmol h À1with 13wt %Pt.In this work,photocatalytic H 2-production activity of the prepared graphene ÀCdS nanocomposites was evaluated under visible-light irradiation using lactic acid as a sacri ﬁcial reagent and Pt as a cocatalyst.The sacri ﬁcial reagent can prevent sul ﬁde photocatalysts from the photocorrosion by providing sacri ﬁcial electron donors to consume the photogenerated holes,and Pt can reduce the overpotential in the production of H 2from water and suppress the fast backward reaction (recombination of hydrogen and oxygen into water)as well.45À47Control experi-ments indicated that no appreciable hydrogen production was detected in the absence of either irradiation or photocatalyst,suggesting that hydrogen was produced by photocatalytic reac-tions on the photocatalyst.Graphene exhibited a signi ﬁcant in ﬂuence on the photocata-lytic activity (Figure 9).Even with a small amount of graphene (0.5À2.5wt %),the H 2-production rate was noticeably in-creased.For CdS alone (GC0),a relatively low photocatalytic H 2-production rate (0.23mmol h À1)was observed as expected due to the rapid recombination of conduction band (CB)electrons and valence band (VB)holes.In the presence of a small amount of graphene (0.5%),the activity of sample GC0.5was slightly enhanced to 0.38mmol h À1,perhaps because the amount of graphene nanosheets was not large enough to e ﬃciently disperse the CdS clusters.When the content was 1.0%(GC1.0),the H 2-production rate reached the highest value of 1.12mmol h À1with apparent quantum e ﬃciency of 22.5%at 420nm (Table 1).In this regard,the photocatalytic activity of sample GC1.0exceeds that ofGC0by a factor of 4.87,and theFigure 7.High-resolution XPS spectra of C1s from (a)GO and (b)GC1.0.Figure 8.FTIR spectra of (top)GC1.0and (bottom)GO.H 2-production rate is signi ﬁcantly greater than that of most semiconductor photocatalysts.This is attributed to two factors:(1)As compared to the pure CdS counterpart (GC0),the larger speci ﬁc surface area of GC1.0o ﬀers more active adsorption sites and photocatalytic reaction centers,which favor an enhanced photocatalytic activity.(2)In the graphene ÀCdS system,gra-phene serves as an acceptor of the electrons generated in the CdS semiconductor and e ﬀectively decreases the recombination probability of the photoexcited electron Àhole pairs,leaving more charge carriers to form reactive species.However,a further increase in the graphene content led to a deterioration of the catalytic performance.In particular,at the graphene content of 40%(sample GC40),the photocatalytic activity dramatically decreased,with an H 2-production rate of only 0.02mmol h À1and quantum e ﬃciency of 0.6%at 420nm (Table 1).It is reason-able because the introduction of a large percentage of blackgraphene led to shielding of the active sites on the catalyst surface and also rapidly decreased the intensity of light through the depth of the reaction solution,which could be called a “shielding e ﬀect ”.48As a consequence,a suitable content of graphene is crucial for optimizing the photocatalytic activity of G ÀCdS nanocomposites.In comparison,no hydrogen was detected when sample G was used as the photocatalyst with Pt as a cocatalyst,suggesting that the bare graphene without CdS clusters is likely not active for photocatalytic H 2production under the experimental conditions studied.On the basis of the above results,a tentative mechanism of the photocatalytic reaction is proposed as illustrated in Figure 10.Under visible-light irradiation,electrons (e À)are excited from the VB to the CB of the CdS semiconductor and then likely transfer in one of three following ways:(1)to Pt deposited on the surface of CdS clusters;(2)to carbon atoms on the graphene sheets;(3)to Pt located on the graphene nanosheets.Eventually,the electrons react with the adsorbed H +ions to form H 2.While the CB edge of CdS is more negative than the reduction potential of H +/H 2,the H 2-production rate is negligible.This can be explained by the rapid recombination rate of CB electrons and VB holes.Once graphene is introduced to the CdS nanoparticles,it can serve as an electron collector and transporter to e ﬃciently separate the photogenerated electron Àhole pairs,e ﬀectively lengthening the lifetime of the charge carriers.Furthermore,the unique features of graphene allow photocatalytic reactions to take place not only on the surface of semiconductor catalysts,but also on the graphene sheet,greatly enlarging the reaction space.4.CONCLUSIONSA high e ﬃciency of the photocatalytic H 2production from water splitting under visible-light irradiation has been achieved over the graphene-CdS photocatalyst synthesized by a solvother-mal method.Graphene nanosheets in the composite enhance the crystallinity and the speci ﬁc surface areas of CdS clusters,and a low amount of graphene can dramatically improve the photo-catalytic activity.The optimal weight percentage of graphene was found to be 1.0wt %,which resulted in a high visible-light photocatalytic H 2-production rate of 1.12mmol h À1and corre-sponding apparent quantum e ﬃciency of 22.5%at 420nm with 0.5wt %Pt as a cocatalyst.The results demonstrate that the unique features of graphene make it an excellent supporting material for semiconductor nanoparticles as well as an electron collector and transporter to separate photogenerated electron Àhole pairs.This work not only demonstrated the potential of graphene as a support for CdS nanoparticles in photocatalytic hydrogen production,but also highlights more generally the potential application of graphene-based materials in the ﬁeld of energy conversion.’AUTHOR INFORMATIONCorresponding Authorgongjr@;jiaguoyu@’ACKNOWLEDGMENTThis work was partially supported by the 973Program (2011CB933401and 2007CB613302),the National Natural Science Foundation of China (21005023,20877061,and51072154),and the Natural Science Foundation of HubeiFigure 10.Schematic illustration of the charge separation and transfer in the graphene ÀCdS system under visible light.The photoexcited electrons transfer from the conduction band of semiconductor CdS not only to the located Pt,but also to the carbon atoms on the graphene sheets,which are accessible to protonsthat could readily transform to H 2.Figure parison of the visible-light photocatalytic activity of samples GC0,GC0.5,GC1.0,GC2.5,GC5.0,GC40,and G for the H 2production using 10vol %lactic acid aqueous solution as a sacri ﬁcial reagent and 0.5wt %Pt as a cocatalyst;a 350W xenon arc lamp was used as the light source.。

改进液相氧化还原法制备高性能氢气吸附用石墨烯

改进液相氧化还原法制备高性能氢气吸附用石墨烯袁文辉1,*李保庆1李莉2(1华南理工大学化学与化工学院,广州510640;2华南理工大学环境科学与工程学院,广州510640)摘要:以液相氧化还原法为基础,并在分散剂十二烷基苯磺酸钠(SDBS)作用下制备得到高质量石墨烯,有效避免了在此过程中石墨烯大量团聚的现象.采用X 射线衍射(XRD)、拉曼光谱(RS)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和原子力显微镜(AFM)等分析手段对石墨烯样品进行了表征.XRD 结果体现了石墨、氧化石墨和石墨烯晶型结构的区别;SEM 和TEM 结果显示石墨烯呈网格状,表面平整,缺陷少;AFM 分析表明样品中单层石墨烯厚度约为1.3nm,同时也存在少许双层结构.BET 测试法得到石墨烯的比表面积高达1206m 2·g -1,考察了石墨烯在高压条件下对H 2的吸附性能.通过对方法改进前后所制备的石墨烯样品进行比较,结果表明,十二烷基苯磺酸钠的加入有效地减小了石墨烯的大量团聚,且得到了高质量的石墨烯.在25和55°C 条件下,高质量石墨烯对氢气的吸附量分别达到1.7%(w )和1.1%(w ),比之前研究结果有了很大提高.关键词:石墨烯;氧化石墨;氢气吸附;氧化还原;超声剥离中图分类号:O647Superior Graphene for Hydrogen Adsorption Prepared by the ImprovedLiquid Oxidation-Reduction MethodYUAN Wen-Hui 1,*LI Bao-Qing 1LI Li 2(1School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou 510640,P .R.China ;2College of Environmental Science and Engineering,South China University of Technology,Guangzhou 510640,P .R.China )Abstract:Graphite oxide (GO)was prepared from liquid oxidation based on Hummers method and the graphene was then prepared using sodium borohydride to reduce the exfoliated graphite oxide by ultrasonication during which moderate sodium dodecyl benzene sulfonate (SDBS)was added into the suspension to reduce the agglomeration among the graphene layers and to obtain a stable graphene suspension.The as-prepared graphene was characterized by X-ray diffraction (XRD),Raman spectroscopy (RS),scanning electron microscopy (SEM),transmission electron microscopy (TEM),and atomic force microscopy (AFM).XRD results show that the crystal structures are different among graphite,graphite oxide,and graphene.SEM and TEM images show that graphene possesses a gridding structure,a smooth surface,and few defects.AFM analysis indicates that the thickness of the single layer graphene is about 1.3nm while there are still a few double layers in the sample.The BET specific surface area of the graphene was about 1206m 2·g -1and its H 2adsorption properties were investigated under high pressure.The samples prepared by liquid oxidation-reduction were compared with that prepared by the improved liquid oxidation-reduction method,which indicates that the addition of SDBS effectively reduces agglomeration among the graphene layers and this generates high quality graphene.The adsorption capacities of H 2on graphene at 25and 55°C reached 1.7%(w )and 1.1%(w ),respectively,which are much higher than that reported previously.[Article]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.-Chim.Sin .2011,27(9),2244-2250SeptemberReceived:April 7,2011;Revised:May 18,2011;Published on Web:June 28,2011.∗Corresponding author.Email:cewhyuan@;Tel:+86-20-87111887.The project was supported by the National Natural Science Foundation of China (20976057).国家自然科学基金(20976057)资助项目ⒸEditorial office of Acta Physico-Chimica Sinica2244袁文辉等:改进液相氧化还原法制备高性能氢气吸附用石墨烯No.91引言当前的能源生产与消费方式使人类的生态环境遭到了严重破坏,而且不断恶化.因此,寻求新的替代型清洁能源已是迫在眉睫.氢能由于具有清洁、高效、可再生等优点被誉为21世纪理想能源,1-3而且越来越受到各国科学家的重视.如何高效、简单地存储氢气并达到美国能源部提出的6.0%(w)吸附量的目标已是当今备受关注的问题.纳米碳材料具有原子质量低、化学稳定性好以及丰富的孔道结构等优点,近年来被广泛应用于氢气存储领域.4-9石墨烯是近年来碳族材料中发现的最新同素异形体,它是由碳原子紧密堆积而成的单层二维蜂窝状晶格结构的晶体薄膜,其厚度只有0.335nm.10由于具有高的比表面积(约2630m2·g-1)、丰富的孔道结构、优良的导电率和易改性的表面化学11-15等特性,自从2004年被英国Manchester大学的Geim小组11发现以来,石墨烯就被广泛应用于气体传感器、锂离子电池、晶体管、氢气存储、催化剂等领域.16-19但是石墨烯的大量制备仍然面临着挑战,主要在于液相还原氧化石墨过程中石墨烯会不可避免地发生团聚,导致得不到高质量的单层石墨烯,同时这也阻碍了石墨烯在吸附领域的应用.因此,寻找有效且能够大量制备单层石墨烯的方法成为当前研究的热点.Srinivas等20用Hummers方法氧化石墨,在水中超声剥离形成氧化石墨烯后,再用水合肼还原制备石墨烯,在压力为1000kPa,温度为-196和25°C下,氢气的吸附量分别达到1.2%(w)和0.1%(w).Ghosh 等21使氧化石墨在高温下热剥离制备石墨烯,在10000kPa和65°C条件下,测得氢气吸附量约2.0% (w)-3.1%(w).Cheng等22的研究结果表明在100 kPa、-196°C和6000kPa、25°C条件下,石墨烯对氢气的吸附量分别达到0.4%(w)和0.2%(w).另外,大量研究16,20-24制备的石墨烯因团聚而导致其石墨烯比表面积远远小于理论比表面积,达不到美国能源部提出的6.0%(w)的氢气吸附量指标.因此,如何得到稳定的石墨烯悬浮液制备高质量单层石墨烯来提高氢气吸附量成为石墨烯在能源领域研究的重要目标.近年来,Lotya25和Xu26等分别用胆酸钠和吡啶酸作为稳定剂制备了石墨烯悬浮液,显示了表面活性剂在制备高质量石墨烯中的优良作用.本文在分散剂十二烷基苯磺酸钠(SDBS)作用下,以氧化石墨为原料,通过NaBH4液相还原法制备得到了高质量石墨烯.采用XRD、Raman、SEM、TEM和AFM等分析手段对石墨烯样品进行了详细表征,BET测试法证明了石墨烯的高比表面积和丰富的孔道结构,用氢气高压吸附实验验证了石墨烯作为一种多孔吸附材料的可观前景.2实验2.1试剂与仪器石墨(约325mesh;45μm;99.8%),高锰酸钾(AR,99.5%),高氯酸钾(AR,≥99.5%),浓硫酸(AR, 95.0%-98.0%),硝酸钠(AR,99.0%),硼氢化钠(AR,≥96.0%)等试剂均购于Alfa Aesar(北京),十二烷基苯磺酸钠(CR,≥85%),双氧水(AR,30%),盐酸(AR,36%-38%),丙酮(AR,99.5%)等购于国药集团化学试剂有限公司,所有溶液均用高纯水配制.科大创新股份有限公司中佳分公司HC-3518高速离心机;德国Bruker公司XRD射线衍射仪;日本Hitachi公司S-3700N型扫描电子显微镜;荷兰Philips-FEI公司Tecnai G2F30S-Twin型透射电子显微镜;美国Veeco Multimode3D原子力显微镜;美国Micromeritics ASAP2010比表面积分析仪;德国Rubotherm磁悬浮天平.2.2氧化石墨的制备采用Hummers法27制备氧化石墨.在冰水浴中装配好500mL的反应瓶,将5g石墨粉和5g硝酸钠与200mL浓硫酸混合均匀,搅拌下加入25g高氯酸钾,均匀后,再分数次加入15g高锰酸钾,控制温度不超过20°C,搅拌一段时间后,撤去冰浴,将反应瓶转移至电磁搅拌器上,电磁搅拌持续24h之后,搅拌下缓慢加入200mL去离子水,温度升高到98°C 左右,搅拌20min后,加入适量双氧水还原残留的氧化剂,使溶液变为亮黄色.然后分次以10000r·min-1转速离心分离氧化石墨悬浮液,并先后用5% HCl溶液和去离子水洗涤直到分离液pH=7.将得到的滤饼真空干燥即得氧化石墨.2.3石墨烯的制备将氧化石墨研碎,称取300mg分散于60mL去2245Key Words:Graphene;Graphite oxide;Hydrogen adsorption;Oxidation-reduction;Ultrasonic exfoliatedVol.27 Acta Phys.-Chim.Sin.2011No.9袁文辉等:改进液相氧化还原法制备高性能氢气吸附用石墨烯Vol.27 Acta Phys.-Chim.Sin.2011No.9袁文辉等:改进液相氧化还原法制备高性能氢气吸附用石墨烯Vol.27 Acta Phys.-Chim.Sin.2011。

基于二氧化钒超材料的双窄带太赫兹吸收器

第 21 卷第 12 期2023 年 12 月太赫兹科学与电子信息学报Journal of Terahertz Science and Electronic Information TechnologyVol.21，No.12Dec.，2023基于二氧化钒超材料的双窄带太赫兹吸收器曹俊豪，饶志明*，李超(江西师范大学物理与通信电子学院，江西南昌330224)摘要：提出一种基于二氧化钒(VO2)超材料的吸收器，由3层结构组成，从上往下分别为2个VO2圆、中间介质层和金属底板。

仿真数据表明，该吸收器有2个很强的吸收峰，分别为4.96 THz 和5.64 THz，相对应的吸收率为99.1%和98.5%。

利用阻抗匹配理论和电场分布进行分析，阐明了吸收的物理机制，并进一步分析了结构参数对吸收率的影响。

所提出的吸收器具有可调谐的特点，能够灵活调控吸收率，为太赫兹波的调控、滤波等功能的实现提供了良好的方案。

该吸收器在图像处理、生物探测和无线通信领域都有潜在的应用。

关键词：太赫兹；超材料；二氧化钒；吸收器中图分类号：TB34 文献标志码：A doi：10.11805/TKYDA2023148 Dual-narrowband THz absorber based on vanadium dioxide metamaterialCAO Junhao，RAO Zhiming*，LI Chao(College of Physics and Communication Electronics，Jiangxi Normal University，Nanchang Jiangxi 330224，China)AbstractAbstract：：A metamaterial absorber based on vanadium dioxide(VO2) is presented. This structure consists of three layers including two vanadium dioxide circles, intermediate dielectric layer, and metalsubstrate from top to bottom. The simulated data shows that the absorber has two strong absorption peaks,at 4.96 THz and 5.64 THz respectively, and the corresponding absorption rates reach 99.1% and 98.5%.The physical mechanism of absorption is clarified by using the impedance matching theory and theelectric field distribution. The effect of the structural parameters on the absorption rate is also analyzed.In addition, the proposed absorber can regulate the absorption rate flexibly, which provides a goodscheme for the realization of terahertz wave regulation, filtering and other functions. Therefore, thisabsorber has potential applications in image processing, biological detection, and wireless communication.KeywordsKeywords：：THz；metamaterial；vanadium dioxide；absorber太赫兹波是指频率为0.1~10 THz的电磁波，相应波长为30 μm~3 mm，它的电磁波谱左侧和右侧分别为电子学和光子学，因此也被称为太赫兹间隙[1]。

graphene enhanced Raman spectra

Can Graphene be used as a Substrate for Raman Enhancement?Xi Ling,†Liming Xie,†Yuan Fang,†Hua Xu,‡Haoli Zhang,‡Jing Kong,§Mildred S.Dresselhaus,§Jin Zhang,*,†and Zhongfan Liu*,††Beijing National Laboratory for Molecular Sciences,Key Laboratory for the Physics and Chemistry of Nanodevices,State Key Laboratory for Structural Chemistry of Unstable and Stable Species,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China,‡State Key Laboratory of Applied Organic Chemistry,College of Chemistry and Chemical Engineering,Lanzhou University,Lanzhou 730000,China,and §Department ofElectrical Engineering and Computer Science,Massachusetts Institute of Technology,Cambridge,Massachusetts 02139ABSTRACT Graphene is a monolayer of carbon atoms packed into a two-dimensional (2D)honeycomb crystal structure,which is a special material with many excellent properties.In the present study,we will discuss the possibility that graphene can be used as a substrate for enhancing Raman signals of adsorbed molecules.Here,phthalocyanine (Pc),rhodamine 6G (R6G),protoporphyin IX (PPP),and crystal violet (CV),which are popular molecules widely used as a Raman probe,are deposited equally on graphene and a SiO 2/Si substrate using vacuum evaporation or solution soaking.By comparing the Raman signals of molecules on monolayer graphene and on a SiO 2/Si substrate,we observed that the intensities of the Raman signals on monolayer graphene are much stronger than on a SiO 2/Si substrate,indicating a clear Raman enhancement effect on the surface of monolayer graphene.For solution soaking,the Raman signals of the molecules are visible even though the concentration is low to 10-8mol/L or less.What’s more interesting,the enhanced efﬁciencies are quite different on monolayer,few-layer,multilayer graphene,graphite,and highly ordered pyrolytic graphite (HOPG).The Raman signals of molecules on multilayer graphene are even weaker than on a SiO 2/Si substrate,and the signals are even invisible on graphite and HOPG.Taking the Raman signals on the SiO 2/Si substrate as a reference,Raman enhancement factors on the surface of monolayer graphene can be obtained using Raman intensity ratios.The Raman enhancement factors are quite different for different peaks,changing from 2to 17.Furthermore,we found that the Raman enhancement factors can be distinguished through three classes that correspond to the symmetry of vibrations of the molecule.We attribute this enhancement to the charge transfer between graphene and the molecules,which result in a chemical enhancement.This is a new phenomenon for graphene that will expand the application of graphene to microanalysis and is good for studying the basic properties of both graphene and SERS.KEYWORDS Graphene,Raman enhancement,substrate,vibrational symmetryINTRODUCTIONRaman spectroscopy is an important and powerful tool in characterizing the structure of materials.However,due to the low scattering cross-section(10-30cm 2molecule -1),the weak intensity of Raman signals result in low sensitivity,which is the reason why the ap-plication of Raman spectroscopy was neglected for many years.1-3The development of techniques for enhancing the Raman signals made Raman spectroscopy more popular in many kinds of research and applications.4-9The techniques are mainly based on resonant Raman scattering (RRS)10or surface-enhanced Raman scattering (SERS).1,7,11For RRS,the excitation wavelength should be resonant with a molec-ular transition.For surface-enhanced Raman spectroscopy (SERS),though there are numerous experimental and theo-retical works on it,there are also many controversies aboutthe mechanism and no complete picture of the enhance-ment mechanism is available even now.12-23Normally,the two widely accepted mechanisms are electromagnetic mech-anism (EM)and chemical mechanism (CM).1EM is based on the enhancement of the local electromag-netic ﬁeld that results in a signiﬁcant increase in the cross section of the Raman scattering.The contribution to the electromagnetic enhancement is mainly due to the surface plasmons excited by the incident light.The enhancement is roughly proportional to |E |4and can get to 108or more,where E is the intensity of the electromagnetic ﬁeld.11,21Compared to EM,CM 11,12,14is based on a charge transfer between the molecule and the substrate.Because of charge transfer,the positive and negative charge in the molecule become more separated,which means the polarizability of the molecule increases,and then the cross section of the Raman scattering increases.CM is usually thought to be a “ﬁrst layer effect”.Some works show that the ﬁrst monolayer of absorbed molecules often exhibits a SERS cross section much larger than that from the second layer.11The enhance-ment of CM is usually 10-102.Additionally,EM is really a long-range effect that requires the substrate to be rough,so*To whom correspondence should be addressed.Professor Jin Zhang,Center for Nanoscale Science and Technology (CNST),College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China.Telephone and Fax:86-10-6275-7157.E-mail:jinzhang@ (J.Z.);zﬂiu@ (Z.L.).Received for review:10/13/2009Published on Web:12/29/2009that a so-called“hot spots”can exist between two particles, while CM is a short-range effect occurring on the molecular scale,which means it requires the molecule to be in contact with the substrate or very close to the substrate,so that charge transfer can easily occur between the molecule and the substrate.In many cases,the two mechanisms coexist. Usually,SERS is mostly based on EM including CM.For SERS,the emphasis on getting strong enhancement is how to get a good substrate.Traditionally,the SERS substrate is based on a rough surface of a noble metal such as Ag,Au,Cu,and so on,24,25which takes advantage of the huge enhancement of EM compared to other enhance-ments.To make a rough metal surface for SERS,the follow-ing methods have been used:(1)electrochemical ways by successive oxidation-reduction cycles as in the work of Fleischmann and co-workers,26(2)depositing a thinﬁlm using vacuum evaporation methods,27,28(3)nanosphere lithography with the assistance of micro-nano-fabrication techniques,29,30(4)preparing size-controlled colloidal nano-particles and depositing a monolayer on the substrate,31-33 (5)assembling a large-area colloidal monolayer of nanopar-ticles on the substrate,34and so on.No matter which method is used,the fabrication process is relatively complex,difﬁcult to control,reproduce,or keep clean,which leads to the low activity of SERS.24Also,the enhanced efﬁciency is often quite different for different metals.Silver is thought to be the best one,but it is expensive and easily oxidized,which will decrease the enhanced efﬁciency.Besides,metals usu-ally have a bad biological compatibility.Therefore,it is necessary to develop new substrates for Raman enhance-ment.To satisfy further requirements,a material,which is cheap and easy to obtain,effective and can be used directly, chemically inert,and biocompatible should be exploited.Graphene,a single sheet from graphite,has the ideal2D structure with a monolayer of carbon atoms packed into a honeycomb crystal plane.Graphene is aromatic and hydro-phobic and has great chemical inertness.People can get graphene by mechanical exfoliation from HOPG or Kish graphite.Graphite is abundant on the Earth’s surface35and we can observe it by optical microscopy on a300nm thick SiO2/Si substrate,which is cheap and convenient for most laboratories.36-38Also,it is biocompatible and has potential bioapplications.39,40As a rising star,it has attracted large interest from both physicists and chemists for its excellent properties both observed and predicted.35,41-43Actually, great progress has been made inﬁnding applications for graphene,from chemical sensors to transistors.44-47Most of them are based on electrical measurements.But its potential as a substrate for Raman enhancement has not been investigated up to now.Here,we analyze the charac-teristics of classical SERS substratesﬁrst and then we consider whether graphene has potential as a substrate for Raman enhancement.The EM mechanism requires a rough surface,metal particles with large curvature,and a surface that can absorb the incident light to produce surface plasmons.The CM mechanism requires the distance between the molecule and the substrate to be below0.2nm and the Fermi level of the metal substrate to symmetrically match with the highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)of the molecule,so that charge transfer can occur from the metal to the molecule or vice versa.1,14Considering graphene,ﬁrst,its surface is relatively smooth in despite ofﬂuctuations that follow from the under lying substrate.48Second,the optical transmission through the graphene surface in the visible range is higher and is more than95%.49Besides,the surface plasmon on graphene is in the range of terahertz rather than in the visible range.50 On the basis of these considerations,graphene does not support EM.On the other hand,for CM graphene has possibilities.One of the most important conditions for CM is charge transfer between the molecule and the substrate. Since previous research has already shown that charge transfer could occur between graphene and some mole-cules,51-54we think that chemical enhancement may occur on the surface of graphene for selected molecules.In fact, in the earlier period,metals with aﬂat surface were used to separate CM from EM for further studying of the CM effect,11 but the cold-deposited method that used to get aﬂat surface of a metalﬁlm is challenging since it requires very low temperature and ultrahigh vacuum.Graphene is an ideal2D plane and the surface of graphene is quiteﬂat.If graphene can be used as a substrate for Raman enhancement based on the chemical mechanism,it will provide an easy method to separate CM from EM for further study.In this work,we use graphene as a substrate for Raman enhancement experiments.Some common molecules used for Raman probes,such as Pc,R6G,PPP,and CV are deposited equally on graphene and a SiO2/Si substrate using vacuum evaporation or solution soaking.By comparing the Raman signals of molecules on monolayer graphene and on a SiO2/Si substrate,we observed that the intensities of the Raman signals on monolayer graphene are much stronger than on the SiO2/Si substrate.For solution soaking,we can observe Raman signals from the molecules on graphene even though the concentration is as low as10-8mol/L,while there is only aﬂuorescence background from the SiO2/Si substrate that is consistent with our previous work.55Fur-thermore,the intensities decrease with an increase in the number of the layers of graphene and eventually the signals can no longer be seen on graphite and HOPG.The Raman enhancement factors of2-17are calculated by taking the signals on the SiO2/Si substrate as references,and they are found to be dependent on the vibrational symmetry of the molecules.This is theﬁrst report about the Raman enhance-ment on the surface of graphene.Though the detailed origin is not so clear,we contribute this phenomenon to chemical enhancement due to the much easier charge transfer be-tween graphene and the molecules.We believe thatfurtherresearch is needed in this direction to understand this special phenomenon.EXPERIMENTAL SECTIONGraphene was prepared by the mechanical exfoliation of Kish graphite (Covalent Materials Corp.)using scotch tape on a SiO 2/Si substrate (300nm thick oxide).Before graphene deposition,the SiO 2/Si substrate was washed clean by ultrasonication in acetone,ethanol,and water in turn,and at the end was cleaned by a Plasma Cleaner.Graphene was characterized by optical microscopy (OM),Raman spectros-copy,and atomic force microscope (AFM).By the color contrast in the OM image (Figure 1a),we can distinguish the relative number of layers as monolayer (1L,pink),few-layer (FL,purple),multilayer (ML,purple close to white),and graphite (G,yellow).In this work,the number of layers is ∼10for FL,50-100for ML,and several hundred or more for graphite.The shape of the Raman G ′-band and the I G /I G ′intensity ratio in the Raman spectra (Figure 1b)and the sample thickness in the AFM image (Figure 1c,d)can give more accurate information about the number of layers.Phthalocyanine (Pc)from Alfa-Aesar,Rhodamine 6G (R6G)from Sigma-Aldrich,Protopphyrin IX (PPP)from Frontier Scientiﬁc,Inc.,and Crystal Violet (CV)from Sinop-harm Chemical Reagent Co.,Ltd.were used directly as received.The chemical structures of these molecules are shown in the Supporting Information.The water used in this work is puriﬁed until the resistivity is 18.2M Ω·cm.We deposited the organic molecules on the surface of graphene using standard thermal evaporation.The pressure during the evaporation is about 10-4Pa.The precision of the thickness measurement is 1Å.Two kinds of thicknesses are used,1and 2Å.For the sample deposited to 1Å,we control the deposition time to about 3s.For the thicknesses of 1and 2Å,the degrees of coverage are all less than amonolayer coverage as shown by many references.56The AFM images of graphene before and after molecular deposi-tion also show a submonolayer coverage that the surface after deposition is as smooth as before (see Supporting Information Figure S2a -d).Meanwhile,for comparison,a bulk HOPG sample was put in the thermal evaporation system at the same time with the graphene sample.STM characterization also shows that the molecules on the sur-face form a submonolayer that we can only ﬁnd when some molecules form a chain on the surface (see Supporting Information Figure S2e,f).We also deposited the organic molecules on the surface of graphene by simply soaking the SiO 2/Si substrate with graphene in the solution of the molecules.Three kinds of molecules with a series of concentrations were used.One is a R6G solution (4×10-6,8×10-7,8×10-8,8×10-9,8×10-10,8×10-11M in water);the second one is a PPP solution (2×10-5,2×10-6,2×10-7,1×10-7,5×10-8,2×10-8M in methanol);the third one is a CV solution (1×10-6M in water).The solutions with different concentrations were prepared by diluting the concentrated solution gradu-ally.The soaking time is 1h for all the solutions.After soaking,the sample with molecules was washed with the corresponding solvent to remove the free molecules and then dried under ﬂowing N 2.Raman spectroscopy was obtained using a Horiba HR800Raman system with three laser lines,a 632.8nm line from He -Ne laser,a 514.5nm line from an Ar +laser (Spectra-Physics model 163-C4205),and a 457.9nm line from an Ar +laser (Melles Griot model 543-AP-A01).The detector is a Synapse CCD detector with thermoelectric cooling to -70°C.An 100×objective was used to focus the laser beam and to collect the Raman signal.The laser power on the sample was 0.5mW for 632.8nm,0.2mW for 514.5nm,and 0.6mW for 457.9nm to avoid possible heating effect bytheFIGURE 1.(a)The optical image of graphene,including single layer (1L),few-layer (FL),multilayer (ML),and graphite (G).(b)The Raman spectra of different numbers of layers of graphene corresponding to (a).The data collection conditions are the same for all spectra.(c)The typical AFM image of graphene.(d)The cross-section analysis of(c).laser.The size of the laser spot is about 1µm.The spectra in comparison were obtained under the same conditions.The intensities of the peaks are obtained by ﬁtting them with a Lorentzian function.RESULTS AND DISCUSSIONTo investigate whether the graphene can be used as a substrate for Raman enhancement,submonolayer mol-ecules were deposited on the sample by vacuum evaporation to ensure an equal distribution on the whole surface.First,the Raman signals of the molecules on graphene and on the SiO 2/Si substrate were compared.Pc with two kinds of thicknesses was deposited;one is thicker,about 2Åand the other one is less,about 1Å.Raman spectra were collected from both graphene and the SiO 2/Si substrate under the same conditions (integration time,laser power,focus,etc.).Figure 2a shows a schematic illustration of the molecules on graphene and on the SiO 2/Si substrate,and the results of the Raman experiment.We found that the signals of Pc on graphene are clearly stronger than that on the SiO 2/Si substrate,no matter which laser excitation was used (Figure 2).The assignments of the peaks are shown in the Support-ing Information (Figure S3),which is in accordance with previous reports.57,58For the sample deposited at 2ÅPc,Pc Raman signals were observed both on graphene and on the SiO 2/Si substrate at each wavelength excitation among 632.8nm (Figure 2b),514.5nm (Figure 2c),and 457.9nm(Figure 2d).We suppose that the absorption spectra of the molecules are not affected by the adsorption of the mol-ecules,whichhasbeenshowntobeverysmallbyothers.59-61The intensities of the Raman signals at 632.8nm excitation are larger than at 514.5and 457.9nm,which is consistent with the fact that the absorption of Pc at 632.8nm is much stronger than at 514.5and 457.9nm.62It is also interesting to note that the Raman signals of Pc on graphene are even stronger than the G-band of graphene (Figure 2b).In any case,the same trend is observed in all three cases and that the signals on graphene are stronger than that on the SiO 2/Si substrate.As the thickness of Pc on the surface decreases to 1Å,the differences of the signals on graphene and on the SiO 2/Si substrate become even larger.At 632.8nm excitation,which is better resonant with Pc,only very weak signals from Pc are observed on the SiO 2/Si substrate,while on graphene the signals are much stronger and well-distin-guished (Figure 2e).At 514.5and 457.9nm excitation,which are outside the absorption range of Pc,the Raman signals at 1340,1543,and 1617cm -1are still distinguish-able on monolayer graphene,in contrast to the signals from Pc on the SiO 2/Si substrate,which are invisible and only the ﬂuorescence background can be seen (Figure 2f,g).In Figure 2f,g,the baselines for graphene and the SiO 2/Si substrate are not at the same level,which is due to the graphene-induced ﬂuorescence quenching.55FIGURE 2.(a)Schematic illustration of the molecules on graphene and a SiO 2/Si substrate,and the Raman experiments.(b -d)are the comparisons of Raman signals of Pc deposited 2Åon graphene (red line)and on the SiO 2/Si substrate (blue line)using vacuum evaporation at 632.8nm excitation (b),514.5nm excitation (c),and 457.9nm excitation (d).(e -g)Similar to (b -d),but depositing less Pc (∼1Å).Except for the peak marked by the star (*)(the 960cm -1peak from Si,and the 1586cm -1peak from graphene),all the peaks are from Pc.The inset in (b)shows the structure ofPc.The situation is similar for R6G and PPP deposited at about 1Å.The assignments of the peaks of R6G and PPP are shown in the Supporting Information (Figure S3).At the resonant wavelength excitation of R6G (Figure 3a)and PPP (Figure 3c),514.5nm,63-65the spectra with strong Raman signals and high signal-to-noise ratio were observed on graphene,while only the ﬂuorescence background was observed when collecting Raman spectra from the SiO 2/Si substrate.At the nonresonant wavelength excitation of R6G (Figure 3b)and PPP (Figure 3d),632.8nm,there is only a ﬂuorescence background from the SiO 2/Si substrate as we expected,but on graphene,though the signal-to-noise ratio is not so good as for the 632.8nm excitation,the signals from the molecules are still visible,which is interesting and exciting.What is more important is that even though the absorption is near to zero at 632.8nm for R6G,63somesignals such as 1312,1360,1512,and 1650cm -1from R6G appeared unexpectedly.For further investigation,solution soaking was used to deposit the molecules.The detailed description about the solution soaking was mentioned in the experimental section.The samples were soaked in a PPP solution with six con-centrations from 2×10-5to 2×10-8M and in a R6G solution with six concentrations from 4×10-5to 8×10-11M.The intensities of the Raman signals of the molecules on monolayer graphene decrease with a decrease in the con-centration for both molecules (Figure 4a,b).The data can be ﬁt well with the BET model (Figure 4c,d)and it is clear to see the intensity of the Raman signal saturate as the con-centration is increased.So,under low concentrations,such as below 8×10-7M for R6G and 2×10-6M for PPP,the degrees of coverage can be thought to besubmonolayer.FIGURE parisons of Raman signals of R6G and PPP deposited on graphene (red line)and on the SiO 2/Si substrate (blue line)using vacuum evaporation at 514.5nm excitation and 632.8nm excitation.(a,b)R6G,(c,d)PPP.(a,c)Excitation at 514.5nm.(b,d)Excitation at 632.8nm.The peak marked by the star (*)is the G-band ofgraphene.FIGURE 4.Raman spectra of PPP (a)and R6G (b)deposited on monolayer graphene by soaking in the solution with different concentrations.For PPP,the concentrations from top to bottom are 2×10-5,2×10-6,2×10-7,1×10-7,5×10-8,and 2×10-8M.For R6G,the concentrations from top to bottom are 4×10-5,8×10-7,8×10-8,8×10-9,8×10-10,and 8×10-11M.(c,d)The corresponding plots of Raman intensity vs concentration.For PPP,we choose the 1615cm -1peak as an example,and for R6G we choose the 1650cm -1peak.The intensities of the peaks are normalized to the signal from silicon at 520cm -1.(e,f)The magniﬁed spectra of PPP under 2×10-8M and R6G under 2×10-5M,respectively.All the spectra are obtained at 514.5nm excitation.The peak marked by the star (*)is the G-band ofgraphene.More importantly,some signals from the molecules can be observed even for concentrations as low as 8×10-10M for R6G (Figure 4e),and 2×10-8M for PPP (Figure 4f).The concentration is comparable with the concentration used in the classical SERS experiments.66,67Compared to the situ-ation on graphene,under all the concentrations both for R6G and PPP no Raman signals were observed on the SiO 2/Si substrate except the ﬂuorescence background,which is the same as in our previous work.55From the experimental data shown above,the much stronger Raman signals of the molecules on graphene compared with that on the SiO 2/Si substrate,and the ap-pearance of the Raman signals for molecules on graphene when soaking in solutions with very low concentrations clearly suggests that a Raman enhancement effect exists on the surface of graphene.As a result of the Raman enhance-ment on the surface of graphene,we can obtain the Raman signals from very few molecules.Also,we can obtain Raman spectra from the molecules at nonresonant wavelength excitations to avoid the ﬂuorescence background that some-times occurs at resonant wavelength excitations.Considering the origin of this special Raman enhance-ment on graphene,as mentioned in the introduction,EM is not likely,while CM is possible.First,the molecules we used are conjugated and macrocyclic (for Pc and PPP).Their basic structures are similar to graphene.When deposited as a submonolayer on graphene,due to the π-πstacking,these aromatic molecules should lie parallel to the surface of graphene,68-70which means that the distance between graphene and the molecules is small.Second,the position of the HOMO and LUMO of the molecules are all located on the two sides of the Fermi level of graphene (see Supporting Information Figure S4.).These observations suggest thatcharge transfer can easily occur between graphene and the molecules,which will induce chemical enhancement.Ad-ditionally,because of the similarity of the chemical structure between the molecules and graphene,the vibrational cou-pling 71between them may be another factor contributing to the Raman enhancement.Under resonant excitation,the signals are attributed to three factors,the number of the molecules,resonant en-hancement,and chemical enhancement.When the amount of molecules is small,the resonant enhancement is insuf-ﬁcient to observe the Raman signals of molecules,and there is no chemical enhancement on the SiO 2/Si substrate,and we cannot see the signals.But on graphene,both resonant enhancement and chemical enhancement exist,which result in observable Raman signals.At the nonresonant wave-length excitation,the signals are attributed to the number of molecules and to chemical enhancement.Because of the fewer molecules and no chemical enhancement on the SiO 2/Si substrate,no signals are observed,while on graphene chemical enhancement still exists,which results in a survival of the signals.Then,tounderstandtheRamanenhancementongraphene further,we compared the Raman signals of molecules on the surface of graphene with different numbers of layers.Figure 5shows the Raman spectra of different molecules on monolayer (blue line),multilayer (green line),and graphite (red line),where molecules were deposited using both vacuum evaporation and solution soaking.For the samples deposited at 1Åthickness by vacuum evaporation (Figure 5a -c),at the corresponding resonant wavelength excitation of Pc,R6G,and PPP,the intensities of the Raman signals from the corresponding molecules decrease with an increase of the number of graphene layers.On the graphite,theFIGURE 5.Raman spectra of Pc (a),R6G (b),and PPP (c)deposited using vacuum evaporation and the Raman spectra of CV (d),R6G (e),and PPP (f)deposited by solution soaking on different surfaces,respectively.The blue line is for spectra obtained on monolayer graphene,the green line is for spectra obtained on multilayer graphene,and the red line is for spectra obtained on graphite.In (e),the spectra are the results after removing the ﬂuorescence background.For all the peaks,except the peaks marked by the star (*),all the peaks are from the indicatedmolecules.signals are usually very weak or invisible.On the surface of HOPG upon which molecules were deposited at the same time with the sample,the signals are hardly observed(see Supporting Information Figure S5.).The trend is analogous to that observed for the samples with deposited molecules obtained by solution soaking(Figure5d-f).In ref72,Wang et al.considered the incident laser and scattered Raman signal interference in between the graphene layers and in this way they explained the G band intensity dependence on the graphene layer number.However,if we apply the same consideration to our system here,we would expect the enhancement factors to oscillate periodically with the graphene thickness.Thus we conclude that the interference effect induced by the different numbers of graphene layers does not make a major contribution to the observed inten-sity.72Regarding the difference of the Raman signals on mono-layer and on multilayer graphene,though we do not know the precise origin of these signals,one possibility is the difference of the surface potential induced by the substrate-induced effects.73The p-doing effect from the SiO2/Si sub-strate will result in an increase in the work function of graphene sheets with thickness,which may affect the ef-ﬁciency of the chemical enhancement.Further work should be done to investigate the origin of this effect.In order to calculate the Raman enhancement factors,we chose the intensity of the Raman signals of Pc on the SiO2/ Si substrate as the normalization reference.The data used here is at632.8nm wavelength excitation.In Figure6,we show the relative intensities of15peaks of Pc on different numbers of layers of paring the signals from monolayer graphene with those from the SiO2/Si substrate, we observed that the Raman enhancement factors of dif-ferent peaks are different and vary from2to17.According to the magnitude of the enhancement,the peaks are clearly distinguished into three classes.The peaks at1140,1340, 1450,1512,and1542cm-1,assigned as A g symmetry vibrations,74are enhanced the most by about15times.The peaks at1082,1182,1189,1312,and1428cm-1,assigned as B3g modes,are enhanced by about5times,74and the peaks at680,722,797,890,and1228cm-1,usually assigned as macrocycle breathing vibrations,74,75are en-hanced relatively less,by about only2times.Also,there are some new peaks appearing,such as766,1159,1405,1479, and1617cm-1(see Figure2e,and the magniﬁed spectra are shown in Supporting Information Figure S7).Most of them are assigned as E modes,which are infrared active (such as766,1159,and1405cm-1),57,74whose intensity is usually weak in other SERS experiments.58Besides,because of the enhancement some overtones in the range1800∼3400 cm-1are visible(see Supporting Information Figure S8).This classiﬁcation of the enhancement factors indicates a vibra-tion symmetry dependence of the Raman enhancement for this effect.On the other hand,with an increase in the number of graphene layers the signals become weaker and weaker.For few-layer graphene,the biggest enhancement factor is about12.For multilayer and more layer graphene, the signals are even weaker than that on the SiO2/Si substrate (Figure6).It should be noted that the magnitude of the enhance-ment,2-17times and the vibration dependence of the enhancement factors are both consistent with the chemical enhancement mechanism.For chemical enhancement,the enhancement of different vibrational modes is usually de-pendent on the geometry of the molecules on the surface.76,77 Here,the enhancement in our system is also vibration dependent,which is related with the surface geometry of the molecule on graphene.The appearances of some new peaks may be due to symmetry-breaking of the molecule. Regarding the vibration dependence of the enhancement of the molecular signal with different vibrational modes and different orientations on the graphene,further investigations are being carried out in our group.These are expected to determine the geometry of the molecule on the surface of graphene by the difference in the Raman enhancement factors for different symmetryvibrations.FIGURE6.The relative Raman intensity of Pc deposited1Åon different surfaces using vacuum evaporation.The different spectral lines represent the different peaks of Pc labeled in the right top corner.The signals on the SiO2/Si substrate are set to“1”.。

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第5期石晓凡，等:石墨烯在防腐防污涂料中的应用研究-107-石墨烯在防腐防污涂料中的应用研究石晓凡，贾新磊(滨州学院化工与安全学院，山东滨州256600)摘要:石墨烯凭借其阻隔性能好、屏蔽性能好以及化学稳定性好等特点，在防腐防污涂料领域得到了广泛应用。

本文综合叙述了近年来石墨烯在防腐、防污涂料中的相关内容，归纳了石墨烯的结构特性，总结了石墨烯在防腐、防污涂料中的应用，整理了石墨烯在涂料方面存在的问题'关键词:石墨烯;结构特性;防腐;防污中图分类号:TQ637文献标识码：A文章编号：1008-021X(2021)05-0107-02Application of Graphene in Anticorrosion and Antifouling CoatingsShi Xiaofan,Jin Xinlei(School of Chemical Enginee/ng and SCety of Binzhou University,Binzhou256600,China)Abstract:Graphene has been widely used in the fieN of anti-corrosion and antifou/ng coatings because of its excellent bc/cr performance,outstanding shielding performance and good chemical stabOty.In thO paper,the related contenO of graphene in anti-cormsion and antifou/ng coa—ngs in recent years are comprehensively described,the structural chamcte时Ucs of graphene aoesummaoczed,theappeccatcon oogoaphenecn antc-co o scon and antcoouecngcoatcngscssummaoczed,and theexcstcngpoobeems oogoaphenecn coatcngsaoesooted out.Key words:graphene；structural properties；corrosion protection；antifou/ng在英国的两位科学家通过众多实验成功分离出了石墨烯后，石墨烯进入了人们的眼界，并且得到了广泛的关注。

中考化学材料英语阅读理解25题

中考化学材料英语阅读理解25题1<背景文章>In recent years, there has been a growing interest in new eco-friendly materials. One such material is a biodegradable plastic that is made from renewable resources. This plastic has several remarkable characteristics.It is strong and durable, yet it can break down naturally over time. This makes it an ideal choice for packaging materials, as it reduces the amount of waste that ends up in landfills. Moreover, it is lightweight, which lowers transportation costs.Another advantage of this material is its versatility. It can be used in a wide range of applications, from food packaging to construction materials. For example, it can be molded into different shapes and sizes for packaging various products. In the construction industry, it can be used to make insulation panels that are energy-efficient.The development of eco-friendly materials like this biodegradable plastic is crucial for a sustainable future. As the world becomes more aware of the environmental impact of traditional materials, the demand for sustainable alternatives is on the rise.1. What is one of the characteristics of the biodegradable plastic?A. Weak and fragile.B. Strong and durable.C. Heavy and bulky.D. Expensive and rare.答案：B。

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谢谢!
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0.3THz的成像效果来自nature materials 2012
• Owing to its high carrier mobility, gapless spectrum and frequency-independent absorption, graphene is a very promising material for the development of detectors and modulators operating in the terahertz region of the electromagnetic spectrum (wavelengths in the hundreds of micrometres), still severely lacking in terms of solid-state devices.
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