管中窥豹—从科学仪器自主研制角度浅谈中国基础研究发展

2023年2月21日习近平总书记主持了二十届中共中央政治局第三次集体学习;并就加强基础研究发表重要讲话,不仅对中国未来科技发展之路具有重要的指导意义,也为普通基础科研工作者指明了前进的方向。在国家大力支持、持续投入和广大科技工作者的长期努力下,我国的科研水平已显著提高,并在某些基础研究领域跻身世界前列。然而,在高精尖科研仪器装备方面,我们仍然很大程度上依赖进口仪器,这成为制约我国科技水平进一步提升的瓶颈。

基于干式制冷的低温扫描探针显微镜研究进展

21世纪以来,扫描探针显微镜(scanning probe microscope,SPM)在微纳尺度形貌表征、物性测量及微纳加工等领域发挥着越来越重要的作用.为了使扫描探针显微镜获得更稳定的运行环境、更高的能量分辨率,人们研发了具备超高真空(ultra high vacuum,UHV)和低温(low temperature,LT)环境的SPM系统(UHV-LT-SPM).目前,大多数的UHV-LT-SPM系统通过向连续流式低温恒温器或低温杜瓦中输送液态氦-4(4He),使SPM的温度达到约4.2 K.然而由于4He元素在自然界中含量低且因需求日益增长,导致液氦价格急剧飙升,严重影响到了4He相关低温设备的正常运行.为应对上述问题,干式制冷技术成为新一代低温技术的发展方向.在此背景下,将干式制冷技术与扫描探针显微镜相结合,搭建干式低温扫描探针显微镜,成为了目前扫描探针仪器领域的研究重点之一.本文主要从扫描探针显微镜系统设计、降温设计、减振方法以及其设备性能等方面,介绍目前已经报道的几种干式LT-SPM系统.最后总结了干式LT-SPM系统目前所遇见的问题和挑战,探讨了该技术未来的发展方向.

低维原子/分子晶体材料的可控生长、物性调控和原理性应用

本文介绍了高鸿钧课题组在物理所20年来的部分代表性工作.研究的主要方向为低维纳米功能材料的分子束外延可控制备、生长机制、物性调控及其在未来信息技术中的原理性应用.从材料的可控制备入手,结合第一性原理的理论计算,阐明材料生长机制和结构与物性的关系,进而实现物性调控和原理性应用.主要内容有:1)纳米尺度"海马"分形结构的形成及其生长机制;2)STM分辨率的提高及最高分辨Si(111)-7×7原子图像的获得;3)固体表面上功能分子的吸附、组装及其机制;4)稳定、重复、可逆的纳米尺度电导转变与超高密度信息存储;5)固体表面上单分子自旋态的量子调控及其原理性应用;6)原子尺度上朗德g因子的空间分辨及其空间分布不均匀性的发现;7)晶圆尺寸、高质量、单晶石墨烯的制备及原位硅插层绝缘化;8)几种新型二维原子晶体材料的可控构筑及其物性调控;9)"自然图案化"的新型二维原子晶体材料及其功能化.这些工作为低维量子结构的构造、物性调控及其原理性应用奠定了基础.

FPGA and computer-vision-based atom tracking technology for scanning probe microscopy

Atom tracking technology enhanced with innovative algorithms has been implemented in this study,utilizing a comprehensive suite of controllers and software independently developed domestically.Leveraging an on-board field-programmable gate array(FPGA)with a core frequency of 100 MHz,our system facilitates reading and writing operations across 16 channels,performing discrete incremental proportional-integral-derivative(PID)calculations within 3.4 microseconds.Building upon this foundation,gradient and extremum algorithms are further integrated,incorporating circular and spiral scanning modes with a horizontal movement accuracy of 0.38 pm.This integration enhances the real-time performance and significantly increases the accuracy of atom tracking.Atom tracking achieves an equivalent precision of at least 142 pm on a highly oriented pyrolytic graphite(HOPG)surface under room temperature atmospheric conditions.Through applying computer vision and image processing algorithms,atom tracking can be used when scanning a large area.The techniques primarily consist of two algorithms:the region of interest(ROI)-based feature matching algorithm,which achieves 97.92%accuracy,and the feature description-based matching algorithm,with an impressive 99.99%accuracy.Both implementation approaches have been tested for scanner drift measurements,and these technologies are scalable and applicable in various domains of scanning probe microscopy with broad application prospects in the field of nanoengineering.

Visualizing the Local Twist Angle Variation within and between Domains of Twisted Bilayer Graphene

Moirésuperlattices in twisted two-dimensional materials have emerged as ideal platforms for engineering quantum phenomena,which are highly sensitive to twist angles,including both the global value and the spatial inhomogeneity.However,only a few methods provide spatial-resolved information for characterizing local twist angle distribution.

Atomic force microscopy investigation of growth process of organic TCNQ aggregates on SiO_2 and mica substrates

Deposition patterns of tetracyanoquinodimethane （TCNQ） molecules on different surfaces are investigated by atomic force microscopy. A homemade physical vapour deposition system allows the better control of molecule deposition. Taking advantage of this system, we investigate TCNQ thin film growth on both SiO2 and mica surfaces. It is found that dense island patterns form at a high deposition rate, and a unique seahorse-like pattern forms at a low deposition rate. Growth patterns on different substrates suggest that the fractal pattern formation is dominated by molecule-molecule interaction. Finally, a phenomenal ＂two-branch＂ model is proposed to simulate the growth process of the seahorse pattern.

Low-Dimensional Forest-Like and Desert-Like Fractal Patterns Formed in a DDAN Molecular System

Two kinds of forest-like and desert-like patterns are formed by thermal evaporation of 4-dicyanovinyl-N, Ndimethylamino-1-naphthalene （DDAN） onto SiO2 substrates. Based on thermal kinetics of the molecules on the substrate the transformation between the forest and desert patterns is due to two factors. The first one is the diffusion length, which is related to the deposition rate, the diffusion potential energy barrier and the substrate temperature. The second one is the strong interaction between the two polarity chemical groups of the molecules, which is beneficial to the formation of branches. Totally different patterns are also found on mica substrates, and are attributed to the anisotropic diffusion and the stronger interaction between DDAN molecules and the mica surface.

Formation of Two-Dimensional AgTe Monolayer Atomic Crystal on Ag(111) Substrate

We report on the formation of two-dimensional monolayer AgTe crystal on Ag(111) substrates. The samples are prepared in ultrahigh vacuum by deposition of Te on Ag(111) followed by annealing. Using a scanning tunneling microscope(STM) and low electron energy diffraction(LEED), we investigate the atomic structure of the samples.The STM images and the LEED pattern show that monolayer AgTe crystal is formed on Ag(111). Four kinds of atomic structures of AgTe and Ag(111) are observed:(i) flat honeycomb structure,(ii) bulked honeycomb,(iii)stripe structure,(iv) hexagonal structure. The structural analysis indicates that the formation of the different atomic structures is due to the lattice mismatch and relief of the intrinsic strain in the AgTe layer. Our results provide a simple and convenient method to produce monolayer AgTe atomic crystal on Ag(111) and a template for study of novel physical properties and for future quantum devices.

Direct measurements of conductivity and mobility in millimeter-sized single-crystalline graphene via van der Pauw geometry

We report the direct measurements of conductivity and mobility in millimeter-sized single-crystalline graphene on SiO2/Si via van der Pauw geometry by using a home-designed four-probe scanning tunneling microscope（4P-STM）. The gate-tunable conductivity and mobility are extracted from standard van der Pauw resistance measurements where the four STM probes contact the four peripheries of hexagonal graphene flakes, respectively. The high homogeneity of transport properties of the single-crystalline graphene flake is confirmed by comparing the extracted conductivities and mobilities from three setups with different geometry factors. Our studies provide a reliable solution for directly evaluating the entire electrical properties of graphene in a non-invasive way and could be extended to characterizing other two-dimensional materials.

Thermo-controllable self-assembled structures of single-layer 4,4"-diamino-p-terphenyl molecules on Au(110)

Here we report the thermo-controllable self-assembled structures of single-layer 4, 4＇＇-diamino-p-terphenyl（DAT）molecules on Au（110）, which are investigated by scanning tunneling microscopy（STM） combined with density functional theory（DFT） based calculations. With the deposition of monolayer DAT molecules on Au（110） and subsequent annealing at 100℃, all DAT molecules adsorb on a（1×5） reconstructed surface with a ladder-like structure. After annealing the sample at about 200℃, STM images show three distinct domains, including DAT molecules on a（1×3） reconstructed surface, dehydrogenated molecules with two hydrogen atoms detached from one amino group（–2H-DAT） on a（1×5）reconstructed surface and dehydrogenated molecules with four hydrogen atoms detached from two amino groups（–4HDAT） on a（1×3） reconstructed surface through N–Au bonds. Furthermore, after annealing the sample to 350℃, STM image shows only one self-assembled structure with –4H-DAT molecules on a（1×3） reconstructed surface. Relative STM simulations of different self-assembled structures show excellent agreements with the experimental STM images at different annealing temperatures. Further DFT calculations on the dehydrogenation process of DAT molecule prove that the dehydrogenation barrier on a（1×5） reconstructed surface is lower than that on（1×3） one, which demonstrate the experimental results that the formation temperature of a（1×3） reconstructed surface is higher than that of a（1×5） one.

Adsorption behavior of Fe atoms on a naphthalocyanine monolayer on Ag(111) surface

Adsorption behavior of Fe atoms on a metal-free naphthalocyanine（H2Nc） monolayer on Ag（111） surface at room temperature has been investigated using scanning tunneling microscopy combined with density functional theory（DFT）based calculations. We found that the Fe atoms were adsorbed on the centers of H2Nc molecules and formed Fe–H2Nc complexes at low coverage. DFT calculations show that Fe sited in the center of the molecule is the most stable configuration, in good agreement with the experimental observations. After an Fe–H2Nc complex monolayer was formed, the extra Fe atoms self-assembled to Fe clusters of uniform size and adsorbed dispersively at the interstitial positions of Fe–H2Nc complex monolayer. Therefore, the H2Nc monolayer grown on Ag（111） could be a good template to grow dispersed magnetic metal atoms and clusters at room temperature for further investigation of their magnetism-related properties.

Construction of monolayer IrTe2 and the structural transition under low temperatures

Bulk iridium ditelluride(IrTe2)is a layered material and is known for its interesting electronic and structural properties,such as large spin-orbit coupling,charge ordering,and superconductivity.However,so far there is no experimental study about the fabrication of monolayer IrTe2.Here we report the formation of IrTe2 monolayer on Ir(111)substrate by direct tellurization method.Scanning tunneling microscope(STM)images show the coexistence of 1/5 phase and 1/6 phase structures of IrTe2 at room temperature.We also obtained STM images showing distorted stripe feature under low temperatures.This stripe feature is possibly induced by the strain between the IrTe2 monolayer and the metal substrate.Density functional theory(DFT)calculations show that the IrTe2 monolayer has strong interaction with the underlying Ir(111)substrate.

A selective control of volatile and non-volatile superconductivity in an insulating copper oxide via ionic liquid gating

Manipulating the superconducting states of high transition temperature(high-Tc)cuprate superconductors in an efficient and reliable way is of great importance for their applications in next-generation electronics.Here,employing ionic liquid gating,a selective control of volatile and non-volatile superconductivity is achieved in pristine insulating Pr2CuO4±δ(PCO)films,based on two distinct mechanisms.Firstly,with positive electric fields,the film can be reversibly switched between superconducting and non-superconducting states,attributed to the carrier doping effect.Secondly,the film becomes more resistive by applying negative bias voltage up to-4V,but strikingly,a non-volatile superconductivity is achieved once the gate voltage is removed.Such phenomenon represents a distinctive route of manipulating superconductivity in PCO,resulting from the doping healing of oxygen vacancies in copper-oxygen planes as unravelled by high-resolution scanning transmission electron microscope and in situ X-ray diffraction experiments.The effective manipulation of volatile/non-volatile superconductivity in the same parent cuprate brings more functionalities to superconducting electronics,as well as supplies flexible samples for investigating the nature of quantum phase transitions in high-Tcsuperconductors.