二维材料及其范德瓦尔斯异质结的高压响应研究进展

裴胜海; 邓晴阳; 王曾晖; 夏娟

doi:10.11858/gywlxb.20210741

二维材料及其范德瓦尔斯异质结的高压响应研究进展

doi: 10.11858/gywlxb.20210741

电子科技大学基础与前沿研究院，四川成都　610054

基金项目: 国家重点研发计划（2019YFE0120300，2018YFE0115500）；国家自然科学基金（62004026，61774029，62004032）；四川省科技厅应用基础研究计划（2021YJ0517，21CXTD0088，2019YFSY0007，2019JDTD0006）

详细信息

作者简介:
裴胜海（1989－），男，博士，博士后，主要从事二维材料的高压拉曼光谱研究. E-mail：peish0000@gmail.com

邓晴阳（1998－），男，硕士研究生，主要从事新型低维纳米材料器件研究. E-mail：deng_qingyang@std.uestc.edu.cn

通讯作者:
夏　娟（1994－），女，博士，研究员，主要从事新型二维半导体材料的电子和光电性能研究. E-mail：juanxia@uestc.edu.cn

中图分类号: O521.2
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出版历程
- 收稿日期: 2021-03-10
- 修回日期: 2021-04-11

Pressure Engineering in Two-Dimensional Materials and vdWs Heterostructures

Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China

摘要

摘要: 高压技术是一种高效、连续、可逆的调控材料结构、电学、光学等物理特性的手段，因此利用压强工程在材料中实现超导态、制备超硬材料等成为高压领域的研究热点。不同于传统的三维体相材料，二维材料及其异质结中独特的层间耦合作用使其具有许多不同于传统材料的物理特性，且这些物理特性极易受到外场影响和调控，使得高压物理成功地拓展到低维材料领域。本文以石墨烯、黑磷、六方氮化硼和过渡金属二硫族化合物等几种典型的二维材料及其异质结为例，概述了二维材料及异质结在高压调控下的结构、电学、声子动力学、光学等方面的响应，并简要讨论这些高压调控下的二维材料在未来电子、光电器件等领域应用的潜力。
- 二维材料 /
- 范德瓦尔斯异质结 /
- 高压
Abstract: Pressure engineering, as an efficient, continuous and reversible method in tuning structure, electric and optical properties, has been extensively used in study of materials. Two-dimensional materials and vdWs heterostructures exhibit intriguing physical properties, thanks to their interlayer coupling, a unique degree of freedom. These interlayer-coupling-mediated properties are extremely sensitive to external perturbations, in particular external pressure, which can effectively tune interlayer spacing and thus modulate interlayer coupling strength. In this article, we review the responses to applied pressure in several representative two-dimensional materials (graphene, black phosphorus, h-BN, transition metal dichalcogenides and vdWs heterostructures). A plethora of phenomena are observed, including pressure-induced phase transition, structural instability, phonon dynamics, metallization, superconductivity etc. Opportunities in designing next-generation functional devices based on pressure engineering in these two-dimensional materials and heterostructures are also discussed.
- two-dimensional materials /
- van der Waals heterostructures /
- high pressure

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球状分子晶体结构一直是近年来的研究热点，如C₆₀^[1]、C₇₀^[2]等。金刚烷（C₁₀H₁₆）是典型的具有高对称性的球状晶体结构分子。常温常压条件下，C₁₀H₁₆的晶体结构是具有Fm3m对称群的立方相（ $\alpha$ 相），每个C₁₀H₁₆分子包括4个CH基团和6个CH₂基团，并且该分子是取向无序的^[3-5]。低温X射线探测结果表明，无序的 $\alpha$ 相在208 K下会转变为有序的四方相^[6]。温度在控制材料的稳定性中起着重要的作用，而压力作为独立的热力学参量，也能够显著改变材料的晶体结构和性质^[7-12]。Ito等^[13]发现，在压力的作用下，材料会发生由无序相向有序相的转变。这一有趣的无序与有序之间的互相转变引起了广大科研工作者的极大兴趣，其中C₁₀H₁₆在高压下的结构和性质引起了人们的关注。

相比于理论研究^[14-15]，对金刚烷的实验研究比较丰富。早在1973年Ito等^[13]就对常温下的金刚烷进行了高压X射线衍射研究，发现0.48 GPa时金刚烷由原来的无序相转变为有序相，而这一有序相正是前人在低温时发现的 $P\bar 4{2_1}c$ 相。由于其实验压力最高仅为0.8 GPa，因此金刚烷在更高压力下的行为仍未知。1979年，Burns等^[16]通过测量金刚烷在常温下的拉曼光谱，也观测到无序相到有序相的转变。2000年，Rao等^[17]将金刚烷的拉曼散射实验压力提高到26 GPa，发现在压力的作用下金刚烷发生了一系列相变，相变压力点分别为0.5、2.8和8.5 GPa，并推测金刚烷在24 GPa可能还会发生一次相变。同年，Vijayakumar等^[18]将金刚烷的同步辐射X射线衍射实验压力提高到25 GPa，他们认为有序的四方相一直稳定存在到12.5 GPa，但是当压力大于16 GPa时，四方相和单斜相都符合其X射线衍射谱的指标化结果，而当压力大于22 GPa时，只有单斜相符合。

综上所述，尽管在高压下已经对金刚烷开展了一些研究，但是其高压相变序列仍存在争议。为此，本研究对金刚烷进行常温拉曼光谱测试，最高压力为25 GPa。通过分析拉曼光谱，验证前人发现的3次相变，并观测是否有其他相变。

1. 实验方法

C₁₀H₁₆样品购于Alfa公司，为高纯粉末。样品的封装在氮气保护的手套箱中完成。高压产生装置采用Mao-Bell式金刚石对顶砧(Diamond Anvil Cell, DAC)，金刚石直径为 $400\;{\text{μ}}{\rm{m}}$ ，封垫材料采用T301钢，预压厚度约 $50\;{\text{μ}}{\rm{m}}$ ，样品腔直径约 $150\;{\text{μ}}{\rm{m}}$ 。由于C₁₀H₁₆样品非常软，可以提供良好的静水压环境，因此样品腔中没有放置其他传压介质。将红宝石与样品一起封装在样品腔中，整个实验过程中的压力标定根据红宝石R1的荧光确定。

高压拉曼实验在吉林大学超硬材料国家重点实验室的共聚焦拉曼散射系统上完成。该系统的激发光源是由相干公司（Coherent Company）生产的固体二极管倍频Nd: Vanadate单纵模激光器，波长为532 nm，最大输出功率可达2.0 W。散射光谱的测量采用Acton Spectra Pro500i光谱仪和液氮制冷的CCD探测仪完成，光谱仪的焦距为 500 mm。实验前，用标准硅片的峰位对系统进行校准。实验过程中，每个拉曼散射谱的采谱时间为120 s。

2. 结果与讨论

常温常压下C₁₀H₁₆分子具有T_d对称群，其不可约表示为： ${\varGamma }$ =5A₁+A₂+6E+7F₁+11F₂^[16]，其中A₁、E、F₂具有拉曼活性。图1为常温常压下测得的C₁₀H₁₆拉曼谱。C₁₀H₁₆的常温常压拉曼谱可分为4个部分。(1)波数在2800～3000 cm^–1范围内，对应于CH、CH₂基团的C–H伸缩振动模式，其中处在2916 cm^–1 ( $\nu _{17}$ )的最强峰为CH基团的C–H伸缩振动峰，2941 cm^–1 ( $\nu _{18}$ )、2893 cm^–1 ( $\nu _{16}$ )及2847 cm^–1 ( $\nu _{15}$ )对应CH₂基团的C–H伸缩振动峰。(2)波数在1350～1500 cm^–1范围内时，处在1434 cm^–1 ( $\nu _{14}$ )的峰对应于CH₂的剪切振动，处在1367 cm^–1 ( $\nu _{13}$ )的峰对应于CH的弯曲振动。(3)波数在700～1300 cm^–1范围内，处在1225 cm^–1 ( $\nu _{12}$ )的最强峰对应CH的弯曲振动，而972 cm^–1 ( $\nu _{8}$ )、951 cm^–1 ( $\nu _{7}$ )、760 cm^–1 ( $\nu _{6}$ )处的峰对应于金刚烷骨架上C-C伸缩振动，1097 cm^–1( $\nu _{9}$ )处的峰对应于C–H的摇摆振动；此外，在1197 cm^–1 ( $\nu _{11}$ )和1193 cm^–1 ( $\nu _{10}$ )处探测到两个微弱的峰，这是之前报道中从未被探测到的。在Rao等^[17]的研究中，虽然没有在常压下观测到这两个峰，但是在无序相向有序相转变之后，1200 cm^–1处出现的新峰实际上就是本研究观测到的 $\nu _{11}$ 和 $\nu _{10}$ ，之所以他们认为只有一个峰，可能是由探测器的分辨率不同导致的。(4)波数在100～700 cm^–1范围内，处在441 cm^–1 ( $\nu _{4}$ )的最强峰对应C–C–C键的变形振动，其他晶格振动都在400 cm^–1以下，其中对于640 cm^–1 ( $\nu _{5}$ )处的峰，前人都未曾在常温常压下观测到，只有Bistričić等^[19]在低温下观测到该峰。表1列出了常温常压下C₁₀H₁₆的拉曼振动峰及其与前人指认的对比。

图 1 常温常压下金刚烷的拉曼谱

Figure 1. Raman spectrum collected at ambient conditions

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表 1 常温常压下金刚烷拉曼振动峰的指认以及与文献的对比

Table 1. Assignments and vibrational frequencies (cm^–1) of observed Raman modes of C₁₀H₁₆ at ambient condition

Frequency/cm^–1					Mode
This work	Ref.[20]	Ref.[17]	Ref.[16]	Assignment	Mode
2941	2944	2943	2940	$\nu _{18}$	C–H stretching mode
2916	2917	2913	2915	$\nu _{17}$	C–H stretching mode
2893	2895	2983	2894	$\nu _{16}$	C–H stretching mode
2847		2845	2847	$\nu _{15}$	C–H stretching mode
	1474
	1450
1434	1437	1440	1435	$\nu _{14}$	CH₂ scissor mode
1367	1371			$\nu _{13}$	CH bending mode
	1315
1225	1223	1225	1221	$\nu _{12}$	CH bending mode
1197				$\nu _{11}$
1193				$\nu _{10}$
1097		1102	1097	$\nu _{9}$	C–H rock mode
974	972	976	971	$\nu _{8}$	C–C stretching mode
950	951			$\nu _{7}$	C–C stretching mode
761	760	759	759	$\nu _{6}$	C–C stretching mode
640				$\nu _{5}$	Lattice mode
441	443	440	442	$\nu _{4}$	C–C–C deformation mode
399				$\nu _{3}$	Lattice mode
187				$\nu _{2}$	Lattice mode
184				$\nu _{1}$	Lattice mode

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| 显示表格

在本高压原位拉曼实验中，最高压力达25 GPa，图2～图5给出了不同波段的拉曼振动峰随压力变化情况以及振动频移随压力的变化。从图2(a)～图5(a)中可以很明显地看出，当压力升至0.6 GPa时，晶格振动和内模振动都发生了明显的变化。首先，在 $\nu _{3}$ 的低频区出现了一个新峰，我们称之为 $\nu _{3}'$ ，而常压时的 $\nu _{4}$ （441 cm^–1）劈裂成两个峰 $\nu _{4{\rm a}}$ 和 $\nu _{4{\rm b}}$ ，同时 $\nu _{5}$ 也劈裂成两个峰 $\nu _{5{\rm a}}$ 和 $\nu _{5{\rm b}}$ 。在910 cm^–1处出现了一个新的弱峰，并且一直保持到本实验的最高压力。除此之外，常压时的 $\nu _{8}$ （974 cm^–1）劈裂成两个峰 $\nu _{8{\rm a}}$ 和 $\nu _{8{\rm b}}$ 。在1107 cm^–1处也出现了微弱的峰，并随着压力的增大而越来越明显；在1111 cm^–1处出现了一个相对较明显的峰；而在常压下非常微弱的两个峰1193 cm^–1和1197 cm^–1也变得非常明显。同时， $\nu _{12}$ 劈裂成两个峰，而 $\nu _{17}$ 劈裂成3个峰。这些变化都说明0.6 GPa时C₁₀H₁₆发生由常温常压下的无序相向有序相（ $P\bar 4{2_1}c$ ）的转变，这个新的有序相称之为 $\beta$ 相。对这个有序的四方相进行群论分析可知，理论上应该有90个独立的拉曼振动峰，而本研究观测到的拉曼振动峰个数远远小于理论数，该现象也出现在很多其他具有范德华力的分子结构中，如HCN^[21]。从本研究获得的拉曼谱上看，有些变化与前人报道很吻合，同时在第一次相变中观测到文献中第2次相变才出现的现象。这种差异可能是由于所采用设备的分辨率不同所导致的。

图 2 298 K、低于25 GPa压力下C₁₀H₁₆的晶格振动峰及其拉曼频移随压力的变化

Figure 2. Lattice vibration modes of solid C₁₀H₁₆ measured to 25 GPa at 298 K (a) and the pressure dependence of the corresponding Raman shift (b)

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图 5 C₁₀H₁₆的C–H伸缩振动峰(2800～3100 cm^–1)(a)及其拉曼频移(b)随压力的变化

Figure 5. Representative Raman spectra of C₁₀H₁₆ of C–H stretching modes (a) and the Raman shift versus pressure (b) in the frequency range of 2800–3100 cm^–1

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图 4 C₁₀H₁₆的内模振动峰(1300～1500 cm^–1)(a)及其拉曼频移(b)随压力的变化

Figure 4. Representative Raman spectra of C₁₀H₁₆ of internal vibrational modes (a) and the Raman shift versus pressure (b) in the frequency range of 1300–1500 cm^–1

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继续加压至1.7 GPa， $\nu _{4}$ 继续劈裂成3个峰，并且随着压力的增大，劈裂越来越明显。同时，在 $\nu _{14}$ 的高频区，出现了一个宽包（1451 cm^–1），随着压力的增大，其强度也随之增强，直到本实验的最高压力，其强度已经超过主峰强度。而在Rao等^[17]的报道中，直到2.7 GPa才探测到这一新峰。晶格振动和内模振动区同时出现新峰说明在该压力点C₁₀H₁₆发生了结构相变，称该新相为 $\gamma$ 相。继续加压至2.5 GPa，在1431 cm^–1处出现了一个宽包，随着压力的增大，其强度越来越强。当压力达到3.2 GPa时，在1103 cm^–1处出现了一个新峰，至此第2次相变结束。分析拉曼谱可以看出， $\beta \to \gamma$ 相的转变是一个缓慢的转变过程，并非瞬间完成。Rao等^[17]认为 $\beta \to \gamma$ 相的转变压力点为2.8 GPa，然而在3.1 GPa时他们也观测到常压时处于1100 cm^–1的峰开始变得不对称，即所谓的劈裂，当压力大于3.1 GPa时拉曼峰才完全表现为 $\gamma$ 相的拉曼峰。因此， $\beta \to \gamma$ 的结构转变过程是在某个压力区间内完成的。

缓慢升高压力至6.3 GPa时，在109 cm^–1位置出现了新的晶格振动峰，如图2(a)中箭头所示，这是前人从未报道过的新峰。这一现象说明C₁₀H₁₆发生了新的结构相变，我们称之为 $\delta$ 相，这并不是Rao等^[17]认为的等结构相变。值得注意的是，在此压力点，内模振动区域并没有新峰出现，说明在该结构相变过程中分子之间先发生变化。直到压力达到7.7 GPa时， $\nu _{9}$ 继续劈裂成3个峰，说明分子内部也发生了变化，至此第3次相变结束。22.9 GPa时，在888 cm^–1处出现了一个新峰，如图3（a）中箭头所示，并且随着压力的增大，其强度也增强，说明C₁₀H₁₆很可能发生了第4次相变，称该新相为 $\varepsilon$ 相。需要说明的是，在Vijayakumar等^[18]对C₁₀H₁₆的高压同步辐射X射线研究中并没有发现第2次和第3次相变，可能是因为X射线探测不到H原子的变化，而第2次和第3次相变极可能是由H原子的变化引起的。相变后的高压结构还需更多的研究来验证，如中子衍射实验、高压理论计算等。另外，从图2(b)～图5(b)中可以看出，所有拉曼振动频移均随着压力的增大发生蓝移，说明各原子间的间距随着压力的增大呈逐渐减小的趋势，并且在本实验压力范围内，没有形成氢键。

图 3 C₁₀H₁₆的内模振动峰(700～1300 cm^–1)(a)及其拉曼频移随压力的变化(b)

Figure 3. Representative Raman spectra of C₁₀H₁₆ of internal vibrational modes (a) and the Raman shift versus pressure (b) in the frequency range of 700–1300 cm^–1

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3. 结　论

对金刚烷的高压拉曼光谱进行了研究，发现金刚烷在压力的作用下经历了 $\alpha \to \beta \to \gamma \to \delta \to \varepsilon$ 的一系列相变，相变压力点分别为0.6、1.7、6.3和22.9 GPa，与前人报道的结果相吻合。首次在拉曼光谱上探测到第3次相变过程中晶格振动峰的变化。具体的高压结构还有待于同步辐射X射线衍射以及高压理论计算来确定。

图 1 石墨烯在高压下的结构和电学响应：(a) DAC压强工程示意图及高压诱导下石墨烯转变为h-金刚石结构的示意图^[25]；(b) 通过高压原位拉曼光谱研究少层石墨烯在常压到51.7 GPa压强范围的结构响应^[22]；(c) 3层石墨烯在室温下的电阻率-压强曲线^[11]；(d) 不同层数的石墨烯G拉曼峰随压强变化的线性频移系数（ $\partial {\omega _{\rm{G}}}/\partial p$ ），其中红方块、蓝圆点和绿三角分别表示传压介质为醇、氩气和氮气^[28]；(e) 双层1.27°转角石墨烯在常压下是金属态，高压下转变为超导态的现象^[30]

Figure 1. Pressure engineering in Graphene: (a) illustration of DAC high-pressure setup and pressure induced phase transition process from trilayer graphene to h-diamond structure^[25]; (b) evolution of Raman spectrum in few-layer graphene, from ambient pressure to 51.7 GPa, showing reversible shift during pressurization and depressurization^[22]; (c) resistance vs. pressure of trilayer graphene measured at room temperature^[11]; (d) linear pressure coefficients ( $\partial {\omega _{\rm{G}}}/\partial p$ ) of graphene G peak (frequency) obtained in alcohol (red squares), argon (blue circles), and nitrogen (green triangle) as a function of thickness^[28]; (e) pressure induced superconductivity in 1.27° twisted bilayer graphene^[30]

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图 2 黑磷在高压下的结构和电学响应：(a) 用高压原位拉曼光谱研究不同层数的黑磷随压强变化的 $A^1_{{\rm g}}$ 、B_2g和 $A^2_{{\rm g}}$ 振动模式的演化^[35]；(b) 体相黑磷中随压强变化的 $A^1_{{\rm g}}$ 、B_2g和 $A^2_{{\rm g}}$ 拉曼模式的半峰全宽（FWHM）演化，虚线表示半导体到拓扑绝缘体相变压强点^[34]；(c) 少层黑磷在高压下从正常绝缘体（NI）转变为二维拓扑狄拉克半金属态（TDSM），及不同层数的黑磷的拓扑相变图，p_C和p_T分别为电子相变点和热稳定点^[39]

Figure 2. Pressure engineering in black phosphorus (BP): (a) the evolution of $A^1_{{\rm g}}$ , B_2g and $A^2_{{\rm g}}$ modes in monolayer, bilayer, trilayer, and bulk BP samples as a function of pressure^[35]; (b) the evolution of full width at half maximum (FWHM) of $A^1_{{\rm g}}$ , B_2g and $A^2_{{\rm g}}$ modes in bulk BP as a function of pressure^[34], (c) top: evolution of band structure in few-layer BP from normal insulator (NI) to a two dimensional topological Dirac semimetal (TDSM) at high pressure, bottom: the pressure-thickness phase diagram of BP, where p_C and p_T are the critical pressures for electronic phase transition and upper limit of thermodynamic stability respectively^[39]

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图 3 h-BN在高压下的结构和光学响应：(a) h-BN在15 GPa下的XRD谱，表明其完成了从h-BN（P6₃/mmc）到w-BN（P6₃mc）的相转变，蓝线和黑线分别为入射方向平行和垂直DAC轴的数据^[51]；(b) h-BN在13 GPa、室温下转变为密堆w-BN的原子结构变化示意图^[50]；(c) 单层（上）和双层（下）h-BN的原子结构对应力的响应^[56]；（d）多层h-BN的缺陷PL峰随压强变化的响应^[56]

Figure 3. Pressure engineering in hexagonal boron nitride (h-BN): (a) the XRD spectra of bulk h-BN at 15 GPa, collected parallel (blue)/perpendicular (black) to the DAC axis^[51]; (b) atomic structure illustration showing phase transition process from h-BN (P6₃/mmc) to w-BN (P6₃mc) at around 13 GPa at room temperature^[50]; (c) schematic diagram for the strain effect on monolayer (top) and bilayer (bottom) h-BN^[56]; (d) evolution of defect-induced PL spectrum as a function of pressure in multi-layer h-BN^[56]

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图 4 MoS₂在高压下的结构、能带及电学响应：(a) MoX₂的2H_c和2H_a原子结构侧视图，X代表S、Se和Te原子^[61]；(b) 单层、双层和3层MoS₂的价带顶（VBM）的能量劈裂以及层间距随压强变化^[70]；(c) 2H-MoS₂的压强-温度相图^[71]

Figure 4. Pressure engineering in molybdenum dichalcogenides (MoX₂): (a) schematic (side view) of 2H_c and 2H_a structures of MoX₂, with X representing S, Se and Te^[61]; (b) band splitting of VBM and interlayer distance vs. pressure for monolayer, bilayer, and trilayer MoS₂^[70]; (c) pressure-temperature phase diagram of bulk 2H-MoS₂^[71]

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图 5 不同对称性的TMDs在高压下的结构、能带及光学响应：(a) 不同层数的2H-MoS₂中的拉曼振动模式A_1g随压强变化^[62]；(b) 体相T_d-WTe₂的特征拉曼峰随压强的变化，激光入射方向平行于a轴^[83]；(c) 体相1T-ReS₂和2H-MoS₂的特征拉曼峰位随压强的变化^[84]；(d) 不同压强范围下双层MoS₂的带间跃迁示意图^[85]

Figure 5. Pressure engineering in transition metal dichalcogenides (TMDs) with different symmetries: (a) evolution of A_1g Raman mode in MoS₂ with different thickness, as functions of pressure^[62]; (b) evolution of Raman spectrum in bulk T_d-WTe₂ with the incident laser illuminating along the a-axes^[83]; (c) evolution of the featured Raman peaks in bulk ReS₂ (E_g, E_g-like, A_g-like) and MoS₂ (E_1g, E_2g, A_g), as functions of applied hydrostatic pressure^[84]; (d) transition path selection rules of bilayer MoS₂ in different pressure ranges^[85]

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图 6 二维范德瓦尔斯异质结在高压下的结构、激子发光、电荷掺杂等响应：(a) WS₂/MoS₂异质结中 $A_1\!\!\!\text{′}$ 和E′振动模式的频率随压强增加的变化，及其与单层WS₂和单层MoS₂的对比^[89]；(b) 双层WSe₂/MoSe₂异质结的PL谱在高压下的响应（左），及第一性原理计算得到的电子能带结构随压强的变化（右）^[10]；(c) Graphene/MoS₂异质结中石墨烯的狄拉克点移动及电荷转移诱导的载流子浓度变化^[91]；(d) 不同层数（1L、2L、3L）的石墨烯/h-BN异质结中的掺杂效率在STM针尖施加压强下的响应对比^[92]

Figure 6. Pressure engineering in 2D vdWs heterostructures: (a) evolution of $A_1\!\!\!\text{′}$ and E′ vibration modes as a function of pressure^[89]; (b) evolution of measured PL spectrum (left) and calculated electronic band structure (right) with pressure in WSe₂/MoSe₂ heterobilayers, from ambient pressure to 2.8 GPa^[10]; (c) the relative shift of the Dirac point with respect to the Fermi level, together with charge-transfer-induced carrier concentration of graphene, as functions of hydrostatic pressure in graphene/MoS₂ heterostructures^[91]; (d) the relative charging efficiency in graphene/h-BN heterostructures with different number of graphene layers (1L, 2L, 3L), as functions of the applied tip force^[92]

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