图 1  频率调制AFM成像原理图及频率偏移-距离曲线$ \Delta f\left(z\right) $和对应的力-距离曲线$ F\left(z\right) $^[50,51]　(a) 一氧化碳修饰的qplus型NC-AFM示意图, 悬臂以振幅A振动, 悬臂振动频率偏移其本征频率$ {f}_{0} $的值$ \Delta f $反映了针尖-样品相互作用力梯度^[50]; (b) $ \Delta f\left(z\right) $曲线; (c) 隧穿电流$ {I}_{t} $(红色)、短程力(绿色)、长程力(深蓝色)和合力(黑色)随距离z变化的示意图^[51]; (d) qPlus型力传感器的光学显微镜照片; (e) qPlus型力传感器的模型示意图^[52]

Figure 1.  Functional principle of frequency modulation AFM and $ \Delta f\left(z\right) $ as well as the corresponding force curve $ F\left(z\right) $^[50,51]: (a) Schematic of a qPlus-based NC-AFM with a CO-tip; the cantilever oscillates at an amplitude of A and the tip-sample force-induced frequency shift of the cantilever from its natural resonance frequency $ {f}_{0} $ is $ \Delta f $^[50]; (b) frequency shifts in FM-AFM; (c) plot of tunneling current $ {I}_{t} $ (red), short range force (green), long range force (dark blue) and total force (black) as a function of distance z between center of front atom and plane defined by the centers of surface atom layer^[51]; (d) optical microscope photograph of qPlus sensor; (e) schematic diagram of qPlus sensor model^[52].

图 2  并五苯分子^[62,79]及六苯并蔻^[77]的化学键分辨NC-AFM图　(a)—(c) Cu(111)表面上并五苯分子的恒高NC-AFM图像、表征示意图及三维力谱; (d) 六苯并蔻结构模型; (e)针尖-样品距离为z = 3.7 Å时在NaCl(2 ML)/Cu(111)上的HBC分子的恒高AFM图像, 扫描振幅A = 0.35 Å; (f) 针尖-样品距离为z = 3.5 Å时在NaCl(2 ML)/Cu(111)上的HBC分子的恒高AFM图像; (g)分子平面上z = 2.5 Å时电子密度分布计算图^[77]

Figure 2.  Pentacene imaged with CO-tip AFM^[62,79] and Hexabenzocoronene model^[77]: (a)–(c) constant height NC-AFM image, characterization schematic diagram and three-dimensional force spectrum of pentaphenyl molecules on Cu(111); (d) hexabenzocoronene model; (e), (f) constant height AFM measurements (A = 0.35 Å) on HBC on Cu(111) at z = 3.7 Å and 3.5 Å; (g) calculated electron density at a distance of 2.5 Å above the molecular plane^[77].

图 3  KPFM原理图及分子局域电荷分布^[47,53]　(a)—(c) 两个未接触的金属拥有共同的真空能级${E_{{\rm{VAC}}}}$, 当两个金属接触时, 其费米能级对齐; 在二者间施加$ V=V ^*$时, 两个接触金属的接触电势差被补偿^[53]. (d) 双势垒隧穿结的简单静电学模型: 针尖和样品间简单静电学等效电路模型, C₀代表针尖-基底电容, C₁代表针尖-吸附原子电容, C₂代表吸附原子-基底电容^[47,53]. (e) Au原子充电前后$ \Delta f\left(V\right) $曲线图($ d$ = 5.8 Å, A = 0.6 Å). 黑色曲线代表数据拟合的抛物线, 黑色箭头指出LCPD的值V ^*[47]

Figure 3.  Schematic illustration of the Kelvin principle and local charge distribution^[53]: (a)–(c) Two different metals which are not connected to each other share the same vacuum level ${E_{{\rm{VAC}}}}$; when the two materials are connected, their Fermi levels align, accompanied by an electron flow to the material with the greater work function; the contact potential difference can be compensated by applying a dc voltage $ V=V ^*$^[53]. (d) Simple electrostatic model for the double-barrier tunnel junction; schematic illustration of the tip and sample system and equivalent circuit of the electrostatic model: ${C_0}$, ${C_1}$ and ${C_2}$ denote the tip–substrate capacitance, the tip– adatom capacitance and the adatom–substrate capacitance, respectively^[47,53]. (e) $ \Delta f\left(V\right) $ spectra measured above a Au atom before and after charge switching (d = 5.8 Å, A = 0.6 Å)^[47]. Solid black lines show parabolic fits to the measured data and the resulting LCPD values V ^* are indicated by arrows^[47].

图 4  针尖诱导的表面化学反应: 前驱体脱羰反应形成环碳C₁₈^[4]. 第一列为前驱体及中间产物的结构示意图. 第二列和第三列分别对应使用CO修饰AFM在$ \Delta z $较小和$ \Delta z $较大时AFM表征图像, z的零点设置为STM模式下I = 0.5 pA, V = 0.2 V. (l)(m)(q)(r)中下方明亮的点对应于CO分子. 第四列和第五列对应体相DFT计算的分子构型. 第二行(f)—(j)、第三行(k)—(o)对应最常见的反应中间产物. 第四行(p)—(t)对应环碳C₁₈. “模拟.远”、“模拟.近”对应同一行的“AFM.远”、“AFM.近”. 所有图像的标尺与图(b)中标尺保持一致

Figure 4.  Precursor and products generated by tip-induced decarbonylation^[4]. Structures are shown in column 1. AFM images (columns 2 and 3) were recorded with a CO-functionalized tip at different tip offsets $ \Delta z $, with respect to an STM set point of I = 0.5 pA, V = 0.2 V above the NaCl surface. (a)–(e) Precursor; ((f)–(j) and (k)–(o)) the most frequently observed intermediates; the bright features in the lower part of (l), (m), (q), and (r) correspond to individual CO molecules; columns 4 and 5 show simulated AFM images based on gas-phase DFT-calculated geometries; (p)–(t) cyclo carbon. The difference in probe height between “sim. far” and “sim. close” corresponds to the respective difference between “AFM far” and “AFM close”. The scale bar in (b) applies to all experimental and simulated AFM images.

图 5  AFM在垂直基底方向纵向操纵针尖-基底原子交换^[16]　(a) 针尖接近(黑色)和远离(红色)基底上的Si原子(右侧白色圆框标识)时的频率偏移$ \Delta f\left(z\right) $曲线, 在这一过程中来自针尖的Sn原子取代了原来在基底上的Si原子; (b) 针尖接近(黑色)和远离(红色)(a)中沉积的Sn原子(右侧黑色圆框标识)时的频率偏移$ \Delta f\left(z\right) $曲线; (c) 在混合半导体表面使用上述操纵方法在低原子浓度处沉积或移除原子, 实现“写”原子标记; (d) 操纵过程中针尖与基底的结构模型. 垂直交换原子的操纵方法包含了针尖和表面间多原子的复杂相互作用. 图中硅Si, 锡Sn, 氢H原子分别用黄、蓝、白球表示, 上半部分代表针尖尖端模型, 下半部分代表表面原子分布模型

Figure 5.  Atom exchange by vertical manipulation of AFM^[16]: (a) Frequency shift $ \Delta f\left(z\right) $ signal upon approach (black) and retraction (red) of the tip over the Si atom marked with a white circle in the left inset image, in this process, the Sn atom from the tip replace the Si atom in the substrate; (b) frequency shift $ \Delta f\left(z\right) $ signal upon approach (black) and retraction (red) of the tip above the Sn atom deposited in (a), pointed out by a black circle (left inset); (c) series of topographic images showing the creation and remove of atomic patterns displaying the symbol of silicon, implementing “write” atomic markers; (d) structural model of tip and substrate during manipulation. These vertical-interchange manipulations involve complex multi-atom contacts between tip and surface. Tin and hydrogen atoms are represented by yellow, blue and white spheres, respectively, and the tip apex and surface models correspond to the atomic arrangements in the upper and lower halves respectively.

图 6  用AFM实现Si, Sn, Pb原子的元素分辨^[40]　(a) 针尖分别与Si(111)表面生长的Sn和Pb单原子层的短程相互作用力探测; (b)使用Si曲线的最小短程力的绝对值($ \left|{F}_{{\rm{S}}{\rm{i}}\left({\rm{S}}{\rm{e}}{\rm{t}}\right)}\right| $)将(a)中力曲线标准化

Figure 6.  Resolution of Si, Sn, Pb by AFM^[40]: (a) Images of a single-atomic layer of Sn and Pb grown, respectively, over a Si(111) substrate; probing short-range chemical interaction forces; (b) the same force curves as in (a), but the curves in each set are now normalized to the absolute value of the minimum short range force of the Si curve(|F_Si(set)|).

图 7  AFM测量在不同基底上操纵不同原子位移的作用力^[8]　(a) 单个吸附物的模拟AFM和STM测量. 振幅A = 30 pm的金属针尖探测金属基底上的单个Co原子或CO分子. 插图显示了针尖在z方向随时间的运动, 针尖距离基底最近时为$z'$, 最远时为$z' + 2 A$. (b) Pt(111)表面原子(灰)及吸附Co原子(红)示意图. 不断降低针尖-样品距离并在最易吸附的方向(x方向)进行连续的扫描, 直至Co原子跳跃到临近吸附位点. (c) 针尖尖端和Co原子之间的力F ^*可以被分解为横向力${F_x}$和垂直力$F_z^*$. 垂直力${F_z}$为$F_z^*$和背景力${F_B}$的和. (d) 测量弹性系数k_z (圆和灰色线)的值, 测得的量是针尖在$z'$与$z' + 2 A$内振动时的时间平均量. (e)—(g) 分别对应由(d)中弹性系数k_z得到的针尖-样品相互作用能U, 垂直力${F'_z}$, 横向力${F_x}$. 线扫描结果被针尖高度z标记. 其中(f)中的红色箭头标识出Co原子跳跃到相邻结合位点. 图(f)和图(g)中彩色线符合s波模型

Figure 7.  Different atoms manipulation on different substrates and tip-substrate interaction by AFM^[8]. (a) Simultaneous AFM and STM measurements of individual adsorbates; an atomically sharp metal tip is oscillating in z with an amplitude A = 30 pm over a flat metal surface on which an individual Co atom or CO molecule is adsorbed. The inset graph shows the tip motion z(t) between its closest distance (z') and farthest distance (z' + 2A) from the sample. The ball models of the surfaces are scaled to match the dimensions of the images in the following panels. (b), (c) Measuring the force to move Co on Pt(111): (b) Schematic top view of the Pt(111) surface atoms (gray) and the adsorbed Co atom (red). In the following panels, constant-height line scans in the direction of easiest adsorbate motion (x direction) were taken at successively reduced tip sample separations until the Co atom hopped to the adjacent adsorption site [red circle in (b)]. (c) The force F ^* between tip apex and the Co atom can be divided into the lateral force ${F_x}$ and the vertical force $F_z^*$. The total vertical force ${F_z}$ is the sum of $F_z^*$ and the background force ${F_{\rm{B}}}$. (d) Stiffness k_z (circles and gray lines). Note that these values are time-averaged over the cantilever oscillation between z = z' and z = z' + 2A. (e)–(g) Tip-sample interaction energy U, vertical force ${F'_z}$, and lateral force ${F_x}$ extracted from the stiffness k_z data in (d). Selected line scans are labeled with the tip height z; the red arrows in (f) indicate the hop of the Co atom to the neighboring binding site. Colored lines in (f), and (g) are fits with the s-wave model.

图 8  AFM针尖操纵DMADAB分子三种表面异构^[10]　(a), (b) Ag(100)表面DMADAB分子的两种异构体的STM图像(标尺为1 nm). (c) 分子动力学模拟的异构化过程内针尖分子相互作用的四个关键过程. (d) 针尖操纵DMADAB分子异构过程中的$ \Delta f\left(z\right) $曲线. 插图展示了DMADAB分子在操纵前(左图)后(右图)的STM图像. 标尺为1 nm, z = 0点设置为二甲氨基结上$ V=- 300$ mV, I = 10 pA. 阶段Ⅰ和Ⅱ对应于针尖接近分子的过程, z由0到–700 pm, 阶段Ⅲ对应于针尖在–700 pm处停留3 s, 阶段Ⅳ和Ⅴ对应针尖缩回的过程, 即从–700 pm到0. (e) 由阶段Ⅰ中的$ \Delta f\left(z\right) $计算得到短程力曲线, 红星标注z的位置为–476 pm, 位于阶段Ⅰ和Ⅱ的交界处, 此时$ \Delta f\left(z\right) $突然下降. 力曲线由Sader–Jarvis方法计算得出^[54,55]

Figure 8.  Three surface isomers of DMADAB molecules and surface reversible isomerization by AFM tip^[10]: (a), (b) STM topographic images DMADAB-1 and DMADAB-2 on the Ag(100) substrate, respectively; scale bars: 1 nm. (c) Four typical states in tip manipulation on DMADAB taken from the SMD simulations on a successful isomerization. (d) The $ \Delta f\left(z\right) $ curve recorded during a successful manipulation on a DMADAB molecule. Insets are STM images of the DMADAB molecule before (left) and after (right) manipulation. Scale bars are 1 nm; z = 0 pm is defined as a tunneling junction height of –300 mV, 10 pA on top of the dimethylamino group. Regions I and II correspond to the tip approaching to the molecule from 0 to –700 pm. Region III is where the tip stays at –700 pm for 3 s; Rregions IV and V correspond to the tip retracting from–700 to 0 pm. (e) The short-range force curve calculated from the $ \Delta f\left(z\right) $ curve in region I; the red star marks the z position at –476 pm, where $ \Delta f\left(z\right) $ suddenly drops at the boundary of regions I and II; the force curve is calculated via the Sader-Jarvis method^[54,55].

图 9  Cu(110)表面NO分子三种吸附构型及在针尖诱导下的相互转化^[11].　(a)—(c) 针尖诱导的直立NO到平卧NO的构型转换过程 (a) Cu(110)表面使用tNO针尖(tilting NO针尖)表征两个直立NO分子的STM图像; (b) 针尖在图(a)中红点位置处接近表面后, 与(a)同样区域的STM图像; (c) 针尖在图(b)中红点位置继续接近表面后, 与(a) (b)同样区域的STM图像. (d), (f) 分别对应(a)$ \to $(b), (b)$ \to $(c)过程中的$ \Delta f\left(z\right) $曲线. (e), (g) 分别对应(a)$ \to $(b), (b)$ \to $(c)过程中的$ U\left(z\right) $曲线, 针尖接近表面和缩回过程分别用红色和蓝色线表示. (h) Cu(110)上直立NO分子附近处tNO针尖探测的$ U(x, z) $分布图. 插图记录了目标分子的STM图像, 蓝绿色线代表x方向, 黑色箭头标明作用于针尖的力$ F(x, z) $分布, 绿色区域标明NO由直立到平卧构型变化的区域. (i)构型变化前原子结构侧视图

Figure 9.  Three adsorption configurations of NO molecules on Cu(110) and $ \Delta f\left(z\right) $, $ U\left(z\right) $ curves of the tip-induced conversion of NO molecules^[11]. (a)−(c) Tip-induced conversion of upright NO into flat-lying NO: (a) STM images of two upright NO molecules on Cu(110) using a tNO tip; (b) STM images of the same area following approach of the tip to the surface over the red point in (a); (c) STM images of the same area after the tip approached the surface over the red point in (b). (d)[(f)] $ \Delta f\left(z\right) $ and (e)[(g)] $ U\left(z\right) $ for (a)$ \to $(b)[(b)$ \to $(c)], the tip approach and retraction are indicated in red and blue, respectively. (h) $ U(x, z) $ map recorded with a tNO tip near an upright NO on Cu(110). Inset shows an STM image of the target. The cyan line represents the x region of the map; black arrows represent force vectors $ F(x, z) $ acting on the tip; green line indicates the region where the NO configuration changes from upright to flat lying. (i) Depicting the side-view atomic structure immediately before conversion.

图 10  偶氮苯分子两种电荷状态下$ \Delta f\left(V\right) $曲线和AFM图像^[13]　(a) 偶氮苯分子上的$ \Delta f\left(V\right) $曲线. 扫描电压V由1 V到3 V. 插图显示偶氮苯的化学结构. (b) V = 0.5 V时的A⁰恒高AFM图像. (c) V = 2.5 V时的A^–1恒高AFM图像. 针尖-样品间距离相对于(b)降低了0.3 Å. (d), (e) 分别对应A⁰和A^–1的模拟恒高AFM图像. 标尺为5 Å. (f), (h) 分别对应A⁰和A^–1的原子模型俯视图. (g), (i)分别对应A⁰和A^–1的化学结构

Figure 10.  $ \Delta f\left(V\right) $ curve and AFM images of azobenzene molecules at two charge states^[13]: (a) $ \Delta f\left(V\right) $ spectrum recorded on top of an azobenzene molecule. Voltage was ramped from 1 to 3 V. The inset shows the chemical structure of azobenzene. (b) Constant-height AFM image of A⁰ at V = 0.5 V. (c) Constant height AFM image of A^–1 at V = 2.5 V, tip-sample distance reduced by 0.3 Å with respect to (b). (d), (e) Simulated AFM images of on-surface A⁰ and A^–1, respectively. All scale bars correspond to 5 Å. (f), (h) Top view of the atomic models of A⁰ and A^–1, respectively. (g), (i) Chemical structures of A⁰ and A^–1, respectively, with wedged bonds representing out-of-plane conformations.

图 11  卟啉分子三种电荷状态下的AFM图像、$ \Delta f\left(V\right) $曲线及键的变化^[13]　(a) 中性卟啉F⁰的化学结构; (b) 负离子卟啉F^–2的化学结构, 红色通路标记的是每种电荷状态下卟啉分子的环形共轭通路; (c), (d) F⁰的恒高AFM图像及Laplace滤波后恒高AFM图像; (e), (f) F^–1的恒高AFM图像及Laplace滤波后恒高AFM图像, 针尖样品的距离比图(c) 和图(d)中大0.5 Å; (g), (h) F^–2的恒高AFM图像及Laplace滤波后恒高AFM图像, 针尖样品的距离比图(c) 和图(d)中大0.4 Å; (i) 卟啉F的$ \Delta f\left(V\right) $谱, 颜色灰度不同代表不同的电荷状态; (j) 卟啉中随电荷状态变化, 键长发生变化的键a, c, l₁, l₂

Figure 11.  $ \Delta f\left(V\right) $ curve and AFM images of porphine molecules at three charge states and the change of the bonds^[13]: Chemical structure of (a) neutral (F⁰) and (b) dianionic (F^–2) porphine, the red path shows the expected annulene-type conjugation pathway for each charge state. constant-height and corresponding Laplace-filtered AFM images of (c) and (d) F⁰, (e) and (f) F^–1, and (g) and (h) F^–2. The constant-height AFM images in (e) and (g) are taken at tip-sample distances larger by 0.5 Å and 0.4 Å, respectively, than the AFM image in (c). (i) $ \Delta f\left(V\right) $ spectrum of F; colored regions indicate the charge states. (j) Highlighted bonds in F. The bonds a, c, ${l_1}$, ${l_{\rm{2}}}$ in porphyrins change with the charge state.

图 12  并五苯分子的AFM电荷状态调控^[15]　(a) 电荷转换循环示意图: 中性$ \to $带负电$ \to $中性$ \to $带正电$ \to $中性, 虚线抛物线显示局部接触电位差(三角形)的变化; (b) AFM操纵最高分子占据轨道(HOMO轨道)分离一个电子的$ \Delta f\left(V\right) $谱; (c) HOMO 轨道添加一个电子的$ \Delta f\left(V\right) $谱; (d) AFM操纵最低分子未占据轨道 (LUMO 轨道) 添加一个电子的$ \Delta f\left(V\right) $谱;（e）LUMO 轨道分离一个电子的$ \Delta f\left(V\right) $谱， 每种情况的偏压变化方向如图中箭头所示; (f) 分子间的横向电荷转移, 插图显示两个临近并五苯分子的恒高AFM图像, 实验零点的参考值$ \Delta f$ = 0.5 Hz, 偏压V = 0 V, $ \Delta f\left(V\right) $曲线为在此零点基础上远离表面6 Å处, 顶部水平轴显示时间依赖性. $ \Delta f\left(V\right) $曲线各部分已由四种不同电荷状态的四条抛物线(虚线)拟合

Figure 12.  Charge state regulation of pentacene by AFM^[15]: (a) Schematic depiction of a closed charge-switching cycle (neutral-negative-neutral-positive-neutral), the dashed parabolas visualize the change of the local contact potential difference (triangles); (b)–(e) experimental manipulation spectra for detaching/attaching a single electron from the highest occupied molecular orbital (HOMO) (b) and to the lowest unoccupied molecular orbital (LUMO) level (d) and the reverse processes (c), (e); the direction of the applied bias ramp is indicated by arrows in each case; (f) lateral charge transfer between individual molecules; the overview AFM image shown in the inset was taken in constant-height mode at a distance determined by a $ \Delta f $ set point of 0.5 Hz at a sample bias of 0 V. The $ \Delta f\left(V\right) $ curve was taken at a distance 6 Å further out from this set point. The time dependence is indicated on the top horizontal axis. The individual segments of the $ \Delta f\left(V\right) $ curve have been fitted by four parabolas (dashed lines) corresponding to the four different charge configurations.