扫描探针显微镜的研制与极端条件下电子关联材料的磁性研究
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摘要
基于量子隧穿原理的扫描隧道显微镜(Scanning Tunneling Microscope,简写为STM)因为具有对导电样品表面原子级别的空间分辨能力,在表面科学与纳米科学领域得到了广泛的应用。例如,人们利用扫描隧道显微镜开展对超导样品表面电子序的研究,在低温环境下对原子进行人工移动和操控,对溶液中样品进行开展化学方面的研究等等。值得注意的是,利用扫描隧道显微镜进行研究的样品在尺寸上常常是较为微小的,很多是在微米甚至纳米级别的。这就对扫描隧道显微镜的应用带来了巨大的挑战,因为要想进行扫描测试首先就得把细长的STM探针对准到微米或者纳米级别样品的表面。
在极端条件如超高真空、低温、极低温和强磁场等环境下面,很多材料表现出了一些非常奇异的物理性质,如超导、量子霍尔效应、德哈斯-范阿尔芬效应和量子相变等等。因此,搭建一台能够在低温强磁场条件下工作的扫描隧道显微镜已成为世界上很多研究小组的研究项目。目前,世界上一些小组已经研制出了能够在磁场环境中工作的扫描隧道显微镜,但是磁场强度不是很高(很多小组的最大可工作磁场在10T左右),这大大限制了STM的应用范围。
磁力显微镜(Magnetic Force Microscope,简写为MFM)是一种利用微悬臂的动力学原理对磁性样品的表面磁结构梯度进行表征的仪器。在研究磁性材料方面得到了广泛的应用,现在几乎成为一种常规的物性表征手段。但低温强磁场条件下面的磁力显微镜依然很少,因为放入磁体环境中会面临很多问题,比如磁体中心的孔径较小,不适合放入外径过大的镜体；磁体本身会带来很多信号的干扰等等。
针对以上的问题和现状,博士期间,在老师的精心指导下我的主要工作有以下几部分：
首先,通过对扫描隧道显微镜对准微小样品测量时所遇到难点的认真分析,我精心设计并开发了一款能够对微小样品进行定位并扫描成像的扫描隧道显微镜系统。利用该STM系统,我们能够实现对十几至几十微米大小样品的一次性精确定位并扫描成像,克服了以前用STM测量小样品方面的诸多不足。由于很多人们感兴趣的样品尺寸往往都比较小,很多是在微米量级的,比如最近比较热门的石墨烯材料、一些重要的微晶样品、电子装置(如磁隧道结、碳纳米管晶体管等)等等；因而我们的这款显微镜具有非常重要的应用方面价值。
目前,世界上用STM测量微小样品常用的方法有：(1)利用马达使得探针盲目地向微小样品步进,经过多次尝试直到探测到指定的样品(2)使用一台具有大范围搜索功能的扫描隧道显微镜。(3)使用扫描隧道显微镜-扫描电子显微镜(STM-SEM)组合镜体,或者扫描隧道显微镜-透射电子显微镜(STM-TEM)组合镜体。无疑,这些方法的缺点是很明显的：或者是比较耗时耗力、效率低(方法1和2),或者是结构比较复杂且兼容性比较差,不能进一步在强磁场等极端环境中测量(方法3)等等。
相比较于上面几种方法,我们设计的这台STM有以下几大优势：(1)可以实现对微小样品的一次性精确定位。镜体采用了分体式扫描结构,对准结构的核心部分是一包含了针、样品和扫描管的软隧道结力学回路。该力学回路上的小扫描管四个外电极负责对样品的两维平面内的扫描,内电极用来调节隧道结的间距。该回路可以自由移动,首先在光学显微镜下面把探针对准到样品上面之后,用银胶粘到一个三角形形状的滑块上面,再移置到镜体的步进扫描管顶端的三个蓝宝石珠构成的平面上。(2)全低压步进,减少了高压步进时对隧道电流信号的干扰。滑块的粗逼近采用了惯性原理和“探-甩”结合机制。先在扫描管上面施加一缓慢变大的电压使得扫描管向前探出,滑块由于静摩擦力作用会跟着前探。倘前放电压大于0.2V则粗逼近过程终止,滑块停止步进。如果电压不大于0.2伏,则滑块快速撤回到自然状态,紧接着给扫描管施加一个尖峰信号,滑块由于惯性作用会向前甩出一小步。粗逼近完成后,通过手动调节针样品之间间距执行细逼近,最后进行扫描成像。(3)兼容性好,可以方便地移植到低温强磁场中测量。由于我们的对准结构(软隧道结力学回路)是独立于步进结构的,因而在把探针对准到了样品之后,可以直接放到低温STM镜体里,然后移植到我们实验室购买的强磁体(18/20T)中进行测量。
我们的实测结果表明该款显微镜具有非常高的定位精度、信号稳定性和测量重复性。高质量的小石墨样品原子图像在一个月之后依然能很容易地扫描得到。实测的隧道电流谱的e指数关系曲线也同时表明了该镜体的隧道结力学回路良好的稳定性。相关的详细镜体结构与实验测量结果得到了审稿人的高度评价,已经发表在了SCI二区期刊Review of Scientific Instruments上面。
此外,利用该镜体高质量的成像能力,在王霁浩的协助下我们迄今第一次发现了电流对石墨烯的晶格点阵具有显著的调制效应。清晰的原子精度图像表明石墨烯样品在未加电流和施加电流之后的晶格点阵有明显差异。
其次,我和实验室已经毕业的博士生李全锋,以及实验室的副研究员侯玉斌还完成了组合显微镜镜体中的扫描隧道显微镜部分(简称为SMA扫描隧道显微镜)在低温强磁场下(磁场高达18/20T)的项目验收工作。我们成功获得了在磁体插件上的前置放大电路的12fA电流分辨率测量和18T强磁场下面石墨原子图像的测量。前者要高于李全锋已经发表在科学仪器评论期刊上相同条件下的测量数据(20fA,迄今世界已发表的最高纪录),远高于世界上别的研究小组的最高电流分辨率(49fA),为进一步在低温强磁场下面进行绝缘体的STM测量奠定了坚实的基础。
组合显微镜是由三种显微镜——扫描隧道显微镜、磁力显微镜和原子力显微镜集成于一个镜体中的兼具三种不同功能的显微镜系统。利用组合显微镜我们可以实现利用三种各不相同的表面表征手段对同一块样品的同一个位置的性质研究。我们这台低温强磁场STM具有以下几大优势：(1)具有目前世界上同等条件下最高的电流分辨能力(12fA),使得在低温磁场下测量绝缘体样品成为可能；(2)20T的石墨原子图形是世界上磁场情况下测量的最高纪录,与当前世界上最高纪录相持平。但是,与现有纪录的18T条件下的纪录相比较,我们的强磁场18T STM具有明显的几大优点：首先,样品测量环境是低温环境,我们的镜体插件是直接浸泡在液氦中的,利用该镜体可以直接进行低温强磁场环境下的样品测量。相比之下,目前最高纪录的18T图像是在室温环境下面的成像。其次,镜体马达步进扫描管较为短粗并且采用了内电极分割技术,使得即使在液氦环境下马达低电压就可行走(最低启动电压为6伏),从而大大减低了低温下面步进信号对隧道电流信号的耦合干扰。第三,我们自创和设计了一系列隔振消音措施和工件,大大降低了外界干扰对测量的影响。对整个磁体装置采用了弹簧悬吊,同时磁体周围包裹了多层吸音海绵；整个扫描隧道显微镜镜体材料一律用无磁材料加工而成,通过弹簧悬吊于磁体插件上,大大降低外界干扰(如液氦挥发,磁场梯度不均匀性等),同时又可随意拆卸,便于操作,兼容性好。
对电流分辨率的测量,我们通过使用一阻值高达100G的大电阻来模拟隧道结电阻,通过改变输入电压来模拟隧道电流变化。在实际的位于磁体插件顶部的前置放大电路的输出端口测量当输入端的隧道电流改变时输出电压的变化。通过高斯拟合最终知道我们的前放最高电流分辨率12fA。验收专家组给予了我们项目极高的评价：“……该系统的成功研制,不但填补了国内在高灵敏、超强磁场极端条件扫描隧道显微镜研制方面的空白,而且在国际上还首次实现了全低压、最高电流分辨率(12fA)……,将对推动材料科学、凝聚态物理等相关学科的发展发挥重要的促进作用……”。
此外,我还对一款能够把样品温度精确降到液氦温度和具有切割样品功能的扫描隧道显微镜镜体进行了初步测试,利用磁体插件的STM电路系统对高序热解石墨进行了成像,并且获得了良好的隧道电流谱线与原子分辨率图像,为下一步在低温强磁场中的切割样品测试奠定了良好的基础。
第三,我利用实验室去年已毕业博士生施益智同学调试成功的组合显微镜镜体中磁力显微镜部分对La(5/8-x)PrxCa3/8MnO3单晶薄膜样品进行了极端环境下的磁畴结构测量。我们在La(5/8-x)PrxCa3/8MnO3薄膜这种材料中非常直观地证实了该体系中确实存在着电子相分离现象,FM相与绝缘相在一定的温度区间共存,并且铁磁性区域的比例随着温度的降低逐渐增大。我们的磁力显微镜的变温测量结果与电输运测量结果非常好地吻合。
磁力显微镜的工作模式采用了振幅调制反馈回路模式。从外源给固定有压阻探针的压电片输入一电压信号使得探针以固定频率振动,当探针逐渐逼近到了磁作用区域时,压阻探针的阻值会因之发生改变,该信号被有源电桥放大。通过PID控制电路维持探针在受力扫描时候的振动幅度不变,PID的调制信号被用来成像。
Scanning tunneling microscope (STM, for short), based on the principle of quantum tunneling in quantum mechanics, has showed its great power in the application of the field of surface and nano sciences because of its spatially atomic resolution. For example, people can use STM to characterize the electronic states of superconductors, manipulate the atoms under low temperature condition, do chemical researches on the samples in the solution and so on. It is noticeable that the size of samples that are studied utilizing STM is usually small; some are in micro-scale or even nano-scale, which brings great challenge for the applications of STM. Because before you attempt to attain a picture with STM, you have to accurately position the thin probe of STM over a small sample (micro-or nano-scale) first.
Once put into extreme conditions, such as ultrahigh vacuum, low temperature, extreme low temperature and high magnetic field and so on, lots of materials exhibit intriguing and unique physical properties (for example, superconductivity, quantum Hall effect, de Hass-van Alphen effect, and quantum phase transition). Thus, it has become an important and imperative project for researchers worldwide to build up a STM that work well at low temperature and in high magnetic field. At present, some research groups have built up STM systems that can work at low temperature and in high magnetic field. But the working magnetic field is not too high (much is about10T or so), which greatly limits application research for samples.
Magnetic force microscope (abbreviated as MFM), based on the dynamic property of microcantilever, is a powerful instrument for measuring distribution of magnetic domain on the surface of magnetic materials. It has been widely used in studying magnetic materials, and has now become a conventional tool for the characterization of physical property. However, the MFM used in high magnetic field is still rare, since many problems would occur when MFM is put into magnet, for example, the bore diameter of the core of magnet is too small to accommodate the microscopy; the magnet itself can bring much signal noise during work and so on.
Taking into consideration the above conditions and questions, during the doctoral period and under my advisor's supervision my work mainly contains the following several parts:
First of all, I have carefully analyzed the issues incurred during positioning a tip over tiny samples using STM, elaborately designed and built up a STM system capable of focusing a tip to a tiny sample accurately just for one time. Using our STM, it is very convenient for us to measure a sample with dimension of microns, overcoming lots of drawbacks that met before in using small samples. Since the intriguing samples that can be investigated using STM are often found to be small. Many of them are at the micron scale. Examples include graphene, some important microcrystalline samples, electronic devices (such as magnetic tunnel junction, carbon nanotube ring transistor, etc.) and so on. Thus, our STM shows significant importance, especially for measuring these samples.
At present, methods that are often used for measuring tiny samples worldwide are several:(1) Blindly approach the tip to the sample and rely on good luck or a large number of trials to find the desired small sample.(2) Using a STM with large area search ability where a piezoelectric motor can move the sample (or tip) in a large range so that the tip can reach and scan most of the sample area.(3) Using a STM-SEM or STM-TEM combo. No doubt, these methods have very obviously weak points:either too much pain is needed (even in vain) or the STM system is more complicated and have bad compatibility (cannot be put into magnet and so on).
Compared to the above introduced methods, our new designed STM have several superiorities:(1) Realizing the positioning once for measuring tiny samples, saving much effort. The STM body takes separate scanning structure. The key part of the focusing structure is a stand-alone soft junction mechanical loop (SJML), in which a tip, small sample and a piezoelectric tube is contained. Two pairs of outer electrodes of the scanner performs scanning in the plane parallel to the sample surface, and the inner electrode of the scanner is used to perform a fine approaching between a tip and sample. The mechanical loop can be freely movable, after we focus the tip over a small sample surface (to facilitate the positioning, we use a thin Pt/Ir tip with the diameter of0.1mm), we fix the SJML to a sliding piece with silver paint, then put the slider to the top isosceles triangle plane formed by three sapphire balls of microscopy body.(2) Fully low voltage operation, thus decreasing noise interruption caused by conventional high approaching voltage. The coarse approaching of the slider piece takes the principle of combination of "forward probing and forward throwing". A gradually increasing voltage signal is added to the approaching piezoelectric tube to make the tube slowly bend forward, which is followed by the sliding piece due to static friction. If the output voltage of preamplifer is bigger than0.2V, the program of coarse approaching automatically stops and the slider also cease approaching. If the output voltage is less than0.2V, a swiftly decreasing signal is applied to the piezotube making the tube restore to original natural state, then a peak signal is quickly added to the tube. Due to the inertia, the sliding piece makes a step forward. After the coarse approaching is finished, we adjust the distance between the tip and sample by hand, and begin scanning when a tunneling current is stable.(3) Excellent performance of compatibility, the structure can be conveniently transplanted to high magnet for measurement. Since the SJML is separate from the approaching structure, it can be moved freely to extreme condition, such as low temperature and high magnet.
Our practical measurement results indicate that the STM has a very high stable positioning resolution, performance and reproducibility. High quality atomic resolution image of HOPG is still obtained after one month for the first measurement. Besides, the feature of tunneling current spectrum also indicates excellent stability of our STM. Related STM structure and measurement data have been highly recommend by reviewers, and have been published in SCI article of the second region-Review of Scientific Instruments.
In addition, with the assistance of J. H. Wang, we have first discovered that current can easily modulate the atomic image of graphene. The comparison between atomic images measured in the condition of adding current to graphene and no current indicates an intriguing distinction.
Second, Q. F. Li who has graduated from our lab last year, the associate researcher Y. B. Hou in our lab and I have accomplished the examination of the research project about the measurement of STM part of three-in-one SMA combo system (includes STM, MFM, and atomic force microscope) at low temperature and high magnetic field (up to18/20T). We have got12fA current resolution of the preamplifier of magnet insert and clear atomic image of HOPG in18T. The former is higher than20fA (the best record) that has been measured by Q. F. Li before and published in a paper of RSI, and much higher than49fA that is the best data measured by other groups in the world hitherto.
In terms of SMA combo system, we can study one position of a sample in the mean while using three different microscopes that perform three different functions. Our low-T high magnetic STM has three superiorities listed in the following:(1) Owning the best current resolution (12fA), making it possible for us to measure insulators in low temperature and high magnet.(2)20T, in which the clear atomic image of HOPG is the highest field in field of SPM, and is the same as the record of a research group in Japan. However, in comparison with the record, our STM have following strong points. First, our measurement condition is low temperature, and magnetic inset is immersed in liquid helium. We can measure samples in low temperature and high magnetic field. In comparison, the image of Japanese group is at room temperature. Second, the motor of our STM take the technique of splitting the inner electrode of piezoelectric tube into two parts, which has greatly decreases the approaching voltage (-6V) and therefore reduces the coupling interruption to the tunneling current during approaching process.(3) We have designed and built up a series of strategies and tools for vibration and noise isolation, greatly decreasing outer interruption (the vibration due to the ununiform of magnetic gradient and the natural evaporation of liquid helium etc.). We hung the whole superconducting magnet with several springs, and wrapped up the magnet with sound-absorbing foam. The SMA-STM microscope is manufactured in terms of nonmagnetic metals, and can be freely hanged to the SMA insert using springs with good convenience for operation and compatibility.
As for the testing of current resolution power of the preamplifier of SMA-STM, we take a large resistor with100G Ohm to simulate real tunneling junction resistor, and modulating different tunneling currents by changing input input voltages. By Gauss curve-fitting method, we finally know that the current resolution power is20fA. The experts of examination of the project have given very positive remarks:"......The successful construction of SMA-STM system, not only fills the gap in the research about STM under high-resolution and high magnetic field conditions in China, but also realizes completely low voltage and the highest current resolution worldwide............it will make a great promotion effect in advancing the development of materials science and condensed matter physics and so on......".
In addition, I have also tested one newly-designed STM that owns the in-situ cutting-sample ability and can exactly reach helium temperature for the sample platform. I have obtained excellent tunneling current spectrum and good atomic resolution image, which has made profound foundation for further cutting-sample measurement by putting STM into Oxford magnet (18/20T).
Third, using the MFM of SMA combo microscopy system that has been tested successfully by Y. Z. Shi who has graduated from our lab last year, I have made measurements for La(5/8-x)PrxCa3/8MnO3thin film at variable temperature and high magnetic field. We have intuitively confirmed that electronic phase separation does exist in La(5/8-x)PrxCa3/8MnO3. What is more important, ferromagnetic and insulating phase coexist at a certain temperature region, and the ferromagnetic zone becomes bigger and bigger with gradually decreasing temperature.
The working principle of MFM takes the style of amplitude modulation feedback circuit. We apply an AC voltage to the piezoelectric plate on which a MFM probe is pasted to vibrate the probe at a certain frequency. The piezoresistive value of the probe makes corresponding changes that is amplified by a bridge circuit once the probe approaches to the regime of magnetic interaction. The vibration amplitude of the probe is maintained constantly via PID feedback circuit during scanning, and the modulation signal of PID circuit is employed to image.
在极端条件如超高真空、低温、极低温和强磁场等环境下面,很多材料表现出了一些非常奇异的物理性质,如超导、量子霍尔效应、德哈斯-范阿尔芬效应和量子相变等等。因此,搭建一台能够在低温强磁场条件下工作的扫描隧道显微镜已成为世界上很多研究小组的研究项目。目前,世界上一些小组已经研制出了能够在磁场环境中工作的扫描隧道显微镜,但是磁场强度不是很高(很多小组的最大可工作磁场在10T左右),这大大限制了STM的应用范围。
磁力显微镜(Magnetic Force Microscope,简写为MFM)是一种利用微悬臂的动力学原理对磁性样品的表面磁结构梯度进行表征的仪器。在研究磁性材料方面得到了广泛的应用,现在几乎成为一种常规的物性表征手段。但低温强磁场条件下面的磁力显微镜依然很少,因为放入磁体环境中会面临很多问题,比如磁体中心的孔径较小,不适合放入外径过大的镜体；磁体本身会带来很多信号的干扰等等。
针对以上的问题和现状,博士期间,在老师的精心指导下我的主要工作有以下几部分：
首先,通过对扫描隧道显微镜对准微小样品测量时所遇到难点的认真分析,我精心设计并开发了一款能够对微小样品进行定位并扫描成像的扫描隧道显微镜系统。利用该STM系统,我们能够实现对十几至几十微米大小样品的一次性精确定位并扫描成像,克服了以前用STM测量小样品方面的诸多不足。由于很多人们感兴趣的样品尺寸往往都比较小,很多是在微米量级的,比如最近比较热门的石墨烯材料、一些重要的微晶样品、电子装置(如磁隧道结、碳纳米管晶体管等)等等；因而我们的这款显微镜具有非常重要的应用方面价值。
目前,世界上用STM测量微小样品常用的方法有：(1)利用马达使得探针盲目地向微小样品步进,经过多次尝试直到探测到指定的样品(2)使用一台具有大范围搜索功能的扫描隧道显微镜。(3)使用扫描隧道显微镜-扫描电子显微镜(STM-SEM)组合镜体,或者扫描隧道显微镜-透射电子显微镜(STM-TEM)组合镜体。无疑,这些方法的缺点是很明显的：或者是比较耗时耗力、效率低(方法1和2),或者是结构比较复杂且兼容性比较差,不能进一步在强磁场等极端环境中测量(方法3)等等。
相比较于上面几种方法,我们设计的这台STM有以下几大优势：(1)可以实现对微小样品的一次性精确定位。镜体采用了分体式扫描结构,对准结构的核心部分是一包含了针、样品和扫描管的软隧道结力学回路。该力学回路上的小扫描管四个外电极负责对样品的两维平面内的扫描,内电极用来调节隧道结的间距。该回路可以自由移动,首先在光学显微镜下面把探针对准到样品上面之后,用银胶粘到一个三角形形状的滑块上面,再移置到镜体的步进扫描管顶端的三个蓝宝石珠构成的平面上。(2)全低压步进,减少了高压步进时对隧道电流信号的干扰。滑块的粗逼近采用了惯性原理和“探-甩”结合机制。先在扫描管上面施加一缓慢变大的电压使得扫描管向前探出,滑块由于静摩擦力作用会跟着前探。倘前放电压大于0.2V则粗逼近过程终止,滑块停止步进。如果电压不大于0.2伏,则滑块快速撤回到自然状态,紧接着给扫描管施加一个尖峰信号,滑块由于惯性作用会向前甩出一小步。粗逼近完成后,通过手动调节针样品之间间距执行细逼近,最后进行扫描成像。(3)兼容性好,可以方便地移植到低温强磁场中测量。由于我们的对准结构(软隧道结力学回路)是独立于步进结构的,因而在把探针对准到了样品之后,可以直接放到低温STM镜体里,然后移植到我们实验室购买的强磁体(18/20T)中进行测量。
我们的实测结果表明该款显微镜具有非常高的定位精度、信号稳定性和测量重复性。高质量的小石墨样品原子图像在一个月之后依然能很容易地扫描得到。实测的隧道电流谱的e指数关系曲线也同时表明了该镜体的隧道结力学回路良好的稳定性。相关的详细镜体结构与实验测量结果得到了审稿人的高度评价,已经发表在了SCI二区期刊Review of Scientific Instruments上面。
此外,利用该镜体高质量的成像能力,在王霁浩的协助下我们迄今第一次发现了电流对石墨烯的晶格点阵具有显著的调制效应。清晰的原子精度图像表明石墨烯样品在未加电流和施加电流之后的晶格点阵有明显差异。
其次,我和实验室已经毕业的博士生李全锋,以及实验室的副研究员侯玉斌还完成了组合显微镜镜体中的扫描隧道显微镜部分(简称为SMA扫描隧道显微镜)在低温强磁场下(磁场高达18/20T)的项目验收工作。我们成功获得了在磁体插件上的前置放大电路的12fA电流分辨率测量和18T强磁场下面石墨原子图像的测量。前者要高于李全锋已经发表在科学仪器评论期刊上相同条件下的测量数据(20fA,迄今世界已发表的最高纪录),远高于世界上别的研究小组的最高电流分辨率(49fA),为进一步在低温强磁场下面进行绝缘体的STM测量奠定了坚实的基础。
组合显微镜是由三种显微镜——扫描隧道显微镜、磁力显微镜和原子力显微镜集成于一个镜体中的兼具三种不同功能的显微镜系统。利用组合显微镜我们可以实现利用三种各不相同的表面表征手段对同一块样品的同一个位置的性质研究。我们这台低温强磁场STM具有以下几大优势：(1)具有目前世界上同等条件下最高的电流分辨能力(12fA),使得在低温磁场下测量绝缘体样品成为可能；(2)20T的石墨原子图形是世界上磁场情况下测量的最高纪录,与当前世界上最高纪录相持平。但是,与现有纪录的18T条件下的纪录相比较,我们的强磁场18T STM具有明显的几大优点：首先,样品测量环境是低温环境,我们的镜体插件是直接浸泡在液氦中的,利用该镜体可以直接进行低温强磁场环境下的样品测量。相比之下,目前最高纪录的18T图像是在室温环境下面的成像。其次,镜体马达步进扫描管较为短粗并且采用了内电极分割技术,使得即使在液氦环境下马达低电压就可行走(最低启动电压为6伏),从而大大减低了低温下面步进信号对隧道电流信号的耦合干扰。第三,我们自创和设计了一系列隔振消音措施和工件,大大降低了外界干扰对测量的影响。对整个磁体装置采用了弹簧悬吊,同时磁体周围包裹了多层吸音海绵；整个扫描隧道显微镜镜体材料一律用无磁材料加工而成,通过弹簧悬吊于磁体插件上,大大降低外界干扰(如液氦挥发,磁场梯度不均匀性等),同时又可随意拆卸,便于操作,兼容性好。
对电流分辨率的测量,我们通过使用一阻值高达100G的大电阻来模拟隧道结电阻,通过改变输入电压来模拟隧道电流变化。在实际的位于磁体插件顶部的前置放大电路的输出端口测量当输入端的隧道电流改变时输出电压的变化。通过高斯拟合最终知道我们的前放最高电流分辨率12fA。验收专家组给予了我们项目极高的评价：“……该系统的成功研制,不但填补了国内在高灵敏、超强磁场极端条件扫描隧道显微镜研制方面的空白,而且在国际上还首次实现了全低压、最高电流分辨率(12fA)……,将对推动材料科学、凝聚态物理等相关学科的发展发挥重要的促进作用……”。
此外,我还对一款能够把样品温度精确降到液氦温度和具有切割样品功能的扫描隧道显微镜镜体进行了初步测试,利用磁体插件的STM电路系统对高序热解石墨进行了成像,并且获得了良好的隧道电流谱线与原子分辨率图像,为下一步在低温强磁场中的切割样品测试奠定了良好的基础。
第三,我利用实验室去年已毕业博士生施益智同学调试成功的组合显微镜镜体中磁力显微镜部分对La(5/8-x)PrxCa3/8MnO3单晶薄膜样品进行了极端环境下的磁畴结构测量。我们在La(5/8-x)PrxCa3/8MnO3薄膜这种材料中非常直观地证实了该体系中确实存在着电子相分离现象,FM相与绝缘相在一定的温度区间共存,并且铁磁性区域的比例随着温度的降低逐渐增大。我们的磁力显微镜的变温测量结果与电输运测量结果非常好地吻合。
磁力显微镜的工作模式采用了振幅调制反馈回路模式。从外源给固定有压阻探针的压电片输入一电压信号使得探针以固定频率振动,当探针逐渐逼近到了磁作用区域时,压阻探针的阻值会因之发生改变,该信号被有源电桥放大。通过PID控制电路维持探针在受力扫描时候的振动幅度不变,PID的调制信号被用来成像。
Scanning tunneling microscope (STM, for short), based on the principle of quantum tunneling in quantum mechanics, has showed its great power in the application of the field of surface and nano sciences because of its spatially atomic resolution. For example, people can use STM to characterize the electronic states of superconductors, manipulate the atoms under low temperature condition, do chemical researches on the samples in the solution and so on. It is noticeable that the size of samples that are studied utilizing STM is usually small; some are in micro-scale or even nano-scale, which brings great challenge for the applications of STM. Because before you attempt to attain a picture with STM, you have to accurately position the thin probe of STM over a small sample (micro-or nano-scale) first.
Once put into extreme conditions, such as ultrahigh vacuum, low temperature, extreme low temperature and high magnetic field and so on, lots of materials exhibit intriguing and unique physical properties (for example, superconductivity, quantum Hall effect, de Hass-van Alphen effect, and quantum phase transition). Thus, it has become an important and imperative project for researchers worldwide to build up a STM that work well at low temperature and in high magnetic field. At present, some research groups have built up STM systems that can work at low temperature and in high magnetic field. But the working magnetic field is not too high (much is about10T or so), which greatly limits application research for samples.
Magnetic force microscope (abbreviated as MFM), based on the dynamic property of microcantilever, is a powerful instrument for measuring distribution of magnetic domain on the surface of magnetic materials. It has been widely used in studying magnetic materials, and has now become a conventional tool for the characterization of physical property. However, the MFM used in high magnetic field is still rare, since many problems would occur when MFM is put into magnet, for example, the bore diameter of the core of magnet is too small to accommodate the microscopy; the magnet itself can bring much signal noise during work and so on.
Taking into consideration the above conditions and questions, during the doctoral period and under my advisor's supervision my work mainly contains the following several parts:
First of all, I have carefully analyzed the issues incurred during positioning a tip over tiny samples using STM, elaborately designed and built up a STM system capable of focusing a tip to a tiny sample accurately just for one time. Using our STM, it is very convenient for us to measure a sample with dimension of microns, overcoming lots of drawbacks that met before in using small samples. Since the intriguing samples that can be investigated using STM are often found to be small. Many of them are at the micron scale. Examples include graphene, some important microcrystalline samples, electronic devices (such as magnetic tunnel junction, carbon nanotube ring transistor, etc.) and so on. Thus, our STM shows significant importance, especially for measuring these samples.
At present, methods that are often used for measuring tiny samples worldwide are several:(1) Blindly approach the tip to the sample and rely on good luck or a large number of trials to find the desired small sample.(2) Using a STM with large area search ability where a piezoelectric motor can move the sample (or tip) in a large range so that the tip can reach and scan most of the sample area.(3) Using a STM-SEM or STM-TEM combo. No doubt, these methods have very obviously weak points:either too much pain is needed (even in vain) or the STM system is more complicated and have bad compatibility (cannot be put into magnet and so on).
Compared to the above introduced methods, our new designed STM have several superiorities:(1) Realizing the positioning once for measuring tiny samples, saving much effort. The STM body takes separate scanning structure. The key part of the focusing structure is a stand-alone soft junction mechanical loop (SJML), in which a tip, small sample and a piezoelectric tube is contained. Two pairs of outer electrodes of the scanner performs scanning in the plane parallel to the sample surface, and the inner electrode of the scanner is used to perform a fine approaching between a tip and sample. The mechanical loop can be freely movable, after we focus the tip over a small sample surface (to facilitate the positioning, we use a thin Pt/Ir tip with the diameter of0.1mm), we fix the SJML to a sliding piece with silver paint, then put the slider to the top isosceles triangle plane formed by three sapphire balls of microscopy body.(2) Fully low voltage operation, thus decreasing noise interruption caused by conventional high approaching voltage. The coarse approaching of the slider piece takes the principle of combination of "forward probing and forward throwing". A gradually increasing voltage signal is added to the approaching piezoelectric tube to make the tube slowly bend forward, which is followed by the sliding piece due to static friction. If the output voltage of preamplifer is bigger than0.2V, the program of coarse approaching automatically stops and the slider also cease approaching. If the output voltage is less than0.2V, a swiftly decreasing signal is applied to the piezotube making the tube restore to original natural state, then a peak signal is quickly added to the tube. Due to the inertia, the sliding piece makes a step forward. After the coarse approaching is finished, we adjust the distance between the tip and sample by hand, and begin scanning when a tunneling current is stable.(3) Excellent performance of compatibility, the structure can be conveniently transplanted to high magnet for measurement. Since the SJML is separate from the approaching structure, it can be moved freely to extreme condition, such as low temperature and high magnet.
Our practical measurement results indicate that the STM has a very high stable positioning resolution, performance and reproducibility. High quality atomic resolution image of HOPG is still obtained after one month for the first measurement. Besides, the feature of tunneling current spectrum also indicates excellent stability of our STM. Related STM structure and measurement data have been highly recommend by reviewers, and have been published in SCI article of the second region-Review of Scientific Instruments.
In addition, with the assistance of J. H. Wang, we have first discovered that current can easily modulate the atomic image of graphene. The comparison between atomic images measured in the condition of adding current to graphene and no current indicates an intriguing distinction.
Second, Q. F. Li who has graduated from our lab last year, the associate researcher Y. B. Hou in our lab and I have accomplished the examination of the research project about the measurement of STM part of three-in-one SMA combo system (includes STM, MFM, and atomic force microscope) at low temperature and high magnetic field (up to18/20T). We have got12fA current resolution of the preamplifier of magnet insert and clear atomic image of HOPG in18T. The former is higher than20fA (the best record) that has been measured by Q. F. Li before and published in a paper of RSI, and much higher than49fA that is the best data measured by other groups in the world hitherto.
In terms of SMA combo system, we can study one position of a sample in the mean while using three different microscopes that perform three different functions. Our low-T high magnetic STM has three superiorities listed in the following:(1) Owning the best current resolution (12fA), making it possible for us to measure insulators in low temperature and high magnet.(2)20T, in which the clear atomic image of HOPG is the highest field in field of SPM, and is the same as the record of a research group in Japan. However, in comparison with the record, our STM have following strong points. First, our measurement condition is low temperature, and magnetic inset is immersed in liquid helium. We can measure samples in low temperature and high magnetic field. In comparison, the image of Japanese group is at room temperature. Second, the motor of our STM take the technique of splitting the inner electrode of piezoelectric tube into two parts, which has greatly decreases the approaching voltage (-6V) and therefore reduces the coupling interruption to the tunneling current during approaching process.(3) We have designed and built up a series of strategies and tools for vibration and noise isolation, greatly decreasing outer interruption (the vibration due to the ununiform of magnetic gradient and the natural evaporation of liquid helium etc.). We hung the whole superconducting magnet with several springs, and wrapped up the magnet with sound-absorbing foam. The SMA-STM microscope is manufactured in terms of nonmagnetic metals, and can be freely hanged to the SMA insert using springs with good convenience for operation and compatibility.
As for the testing of current resolution power of the preamplifier of SMA-STM, we take a large resistor with100G Ohm to simulate real tunneling junction resistor, and modulating different tunneling currents by changing input input voltages. By Gauss curve-fitting method, we finally know that the current resolution power is20fA. The experts of examination of the project have given very positive remarks:"......The successful construction of SMA-STM system, not only fills the gap in the research about STM under high-resolution and high magnetic field conditions in China, but also realizes completely low voltage and the highest current resolution worldwide............it will make a great promotion effect in advancing the development of materials science and condensed matter physics and so on......".
In addition, I have also tested one newly-designed STM that owns the in-situ cutting-sample ability and can exactly reach helium temperature for the sample platform. I have obtained excellent tunneling current spectrum and good atomic resolution image, which has made profound foundation for further cutting-sample measurement by putting STM into Oxford magnet (18/20T).
Third, using the MFM of SMA combo microscopy system that has been tested successfully by Y. Z. Shi who has graduated from our lab last year, I have made measurements for La(5/8-x)PrxCa3/8MnO3thin film at variable temperature and high magnetic field. We have intuitively confirmed that electronic phase separation does exist in La(5/8-x)PrxCa3/8MnO3. What is more important, ferromagnetic and insulating phase coexist at a certain temperature region, and the ferromagnetic zone becomes bigger and bigger with gradually decreasing temperature.
The working principle of MFM takes the style of amplitude modulation feedback circuit. We apply an AC voltage to the piezoelectric plate on which a MFM probe is pasted to vibrate the probe at a certain frequency. The piezoresistive value of the probe makes corresponding changes that is amplified by a bridge circuit once the probe approaches to the regime of magnetic interaction. The vibration amplitude of the probe is maintained constantly via PID feedback circuit during scanning, and the modulation signal of PID circuit is employed to image.
引文
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