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Science Advances reports ICQM Faculty members Nanlin Wang and collaborators’ work on uncooled, ultrabroadband photosensitivity from visible to terahertz wavelengths based on 1T-TaS2 CDW system.
《Science Advances》报道王楠林研究组关于电荷密度波材料室温下超宽光谱光电响应研究进展

The ability to convert light into electronic signal is critical in photoelectronics.The effect has broad applications ranging from imaging,communication, quantuminformation to space science. However, highsensitivity to light over a broad spectral range, especially in low frequenciesdown toTHz at room temperature, is particularlyrare but meritorious. Conventional semiconductors, such as silicon,gain the photoresponse properties through the single-particle excitationsacross the band gap and are transparent to the energy below thegap to THz photons. On the other hand, the photoresponse propertiesof simple metals or very narrow-gap semiconductors suffer seriouslyfrom free-carrier screening and fast-quenching effect, especially atelevated temperatures. Solids with collective electronic states,on the contrary, can have an electronic response in a collective fashionand behave quite differently from the single-particle excitations.

 

Charge density wave (CDW) is one of the most studied collectiveelectronic states in solids. Different fromordinary metals in which onlysingle-particle excitations exist, CDWmaterials also have collective excitationsbeing referred to as an amplitude mode and a phase mode.Amplitude excitation behaves as optical phonons and is not expectedto have a direct effect on the electric transport properties, while phaseexcitation corresponds to the translationalmotion of theCDWcondensateand can have a marked effect on charge transport properties. Thephasemode is usually pinned at finite frequency. By applying a dc electricfield, the phase mode can be driven into a current-carrying state, beingusually referred to as sliding CDW, leading to nonlinear I-Vcharacteristics as long as the field is higher than a threshold value.

 

As one of the most studied CDWcompounds of 2D transition-metal chalcogenides, 1T-TaS2 hosts multiple equilibrium states arising fromthe interplay between electron correlation, lattice strain and Fermi surface instability. Upon coolingacross ~550 K, the system transfers from a simple metal into an incommensurate (IC-) CDW state withan associated modulations of lattice distortions. The distortions sharpen to formstar-shaped polaronclusters and become nearly commensurate (NC-) below Tc (~350 K), being referredto as the phase of NC-CDW. Further cooling leads to another first-order phase transition near 183 K,below which the system becomes a gapped commensurate (C-) CDW state. These variations have made1T-TaS2 an ideal playground for investigating CDW dynamicsand novel metastable statesvia external manipulations.

 

Recently, Dr Dong Wu in Prof Nan Lin Wang’s group at ICQM of Peking University and collaborators reported that incident illuminationscan induce significant changes in the electronic properties of 1T-TaS2when the bias voltage is close but slightly lower than the threshold that leads to the charge density wave slip. A very large photoelectric response effectwith current responsivities on the order of ~1 AW−1 at room temperatureis induced and observed over a wide range of photon energy from ultraviolet to terahertz. The revealed charactershave direct relations with the inherent collective electronic dynamics of the CDW state and the resultsmay extend well beyond the 1T-TaS2 material case. The advantages of uncooled and ultrabroadbandphotoresponse make this 2Dchalcogenide highly attractive forexploring more efficientphotoelectronics, such as novel memory devices,photodetectors and spectroscopy, from bothexperimental and theoretical perspectives.

 

Dr. Dong Wu in Prof. Nanlin Wang’s group is the leading author of this work. The paper was published on Science Advances on August 1, 2018 (http://advances.sciencemag.org/content/4/8/eaao3057).Prof. Yongchang Ma at Tianjin University of Technology, Prof. Ziran Zhao’s group at Tsinghua University and Prof. Jian Wei’s group at ICQM collaborated on this project. The work was supported by the National Science Foundation of China, the National Key Research and Development Program of China, and the Collaborative Innovation Center of Quantum Matter in Beijing, China.

 

 

Figure. The photoresponse properties of 1T-TaS2 at room temperature in Near commensurate CDW phase. A: Thephotoresponse of dc electrical current of a device for voltage sweep up mode. B: The typical current switching effects. C: The photoresponse spectra under various biases and illuminations.The currentresponse as the function of incident intensities was investigated at illumination of λ = 1550 nmfor applied voltages below the initial threshold of dark. D: The ultrabroadbandphotoresponsivity.

  近日,北京大学量子材料中心王楠林研究组的吴东博士及合作者在二维电荷密度波(CDW)材料1T-TaS2 中发现室温下超宽光谱的光电响应现象。他们研究了室温下处在近公度CDW状态的1T - TaS2材料系统,发现当外加偏压(电场)接近CDW滑移的阈值附近时,材料对从可见光到太赫兹宽广能量区间的光辐照变的非常敏感。这一发现为实现非制冷、超宽带(包含太赫兹波段)和灵敏的光电探测研究开辟了一条新的途径。该工作以研究长文(Research Article)形式发表于近期的Science子刊上:Science Advances, aao3057(2018)。吴东、王楠林是该工作的共同通讯作者。

  将光信号转换为电信号的能力在光子学领域极为重要。这种效应在成像,通讯,量子信息以至到空间科学领域都有广泛的应用。但是,基于同一器件的超宽光谱,特别是包含远红外和太赫兹波段的光电探测一直是这一领域的难点。受电子能带结构的限制,传统半导体对小于其能隙能量的光无法直接响应。例如硅基光电探测器,通过单粒子激发穿过带隙(~1.1 eV)来实现光响应,因此对于低于其带隙能量的光谱如远红外和太赫兹是透明的。另一方面,对于普通金属,虽然其带隙为零,但由于存在大量的自由载流子,往往直接屏蔽了可能发生的光激发信号,从而不适合作为光电探测器材料。一般金属和半导体的电子激发属于单粒子激发范畴。

  与普通金属和半导体不同,一些特殊物态如超导和电荷密度波不仅有单粒子激发同时还存在集体激发模式。电荷密度波是固体中研究最多的集体凝聚现象之一,物理上具有两种不同的集体激发模式,即振幅模式(Amplitude mode)和相位模式(Phase mode)。振幅激发表现为电荷密度的纵向振动(CDW 振幅的变化),类似于光学支声子的激发,不会对电荷输运性质产生直接影响。而相位激发模式对应于CDW整体的平移,对应于电荷的流动。在没有任何缺陷或杂质存在的理想情况,这一平移运动不受阻碍,应具有零电阻特性。在实际材料中,由于杂质、缺陷的自然存在,CDW被钉扎在有限的频率上从而具有有限的电阻。但当施加电场超过相位激发模式的阈值时,电荷密度波材料则会出现显著的非线性电流电压行为,表现为电阻的急剧下降,通常称为CDW的滑移。

  1T-TaS2是具有代表性的二维CDW材料。电子关联效应以及电声子相互作用使得这一体系存在丰富的物相结构。随着温度降低,其先后经历从简单金属到非公度-CDW相(相变温度~550K),近公度-CDW相(相变温度~350K),最终进入公度-CDW绝缘相(< 183K)。1T-TaS2 电学性质对外加电场呈现显著的非线性依赖关系。吴东等研究发现,当外加偏压小于导致电荷密度波滑移的阈值时,如果外加光激发,则会导致电阻态的变化,即提前诱导了phase mode的滑移。与此对应,相当于有一个巨大的光电响应效应,并且在从紫外到太赫兹的宽广能量范围都存在,该效应可用于制造宽广能量区域的光电响应探测器件。

  

  

图. 1T-TaS2 超宽光谱光电响应特性。A. 光照对I-V曲线的影响。B. 光电流的开关效应。C. 不同偏压下,器件在λ = 1550 nm辐照的光电流响应。D. 阈值电压附近,不同波段的光响应度(R)的大小。

 

  天津理工大学马永昌教授、清华大学赵自然教授研究组和量子材料中心危健研究组参与了该项工作的合作研究。该工作得到了国家自然科学基金、国家重大科学研究计划及“2011计划”量子物质科学协同创新中心的支持。