Colloquium: Time-resolved scanning tunneling microscopy

Scanning tunneling microscopy has revolutionized our ability to image, study, and manipulate solid surfaces on the size scale of atoms. One important limitation of the scanning tunneling microscope (STM) is, however, its poor time resolution. Recording a standard image with a STM typically takes about a fraction of a second for a fast scanning STM to several tens of seconds for a standard STM. The time resolution of a STM can, however, be significantly enhanced by at least several orders of magnitude. Here various methods are reviewed that are applied in order to significantly improve the time resolution of STM. These methods include high-speed or video STM, atom-tracking STM, and monitoring of the open feedback loop current or closed feedback loop $z$-piezo-voltage signals as a function of time. An analysis of the time-resolved STM data allows one to map out the potential landscape of the system under study.

Figure 1

(Color online) Schematic representations of the two scanning modes in STM: (a) constant-current topography mode and (b) constant-height topography mode. In (a) the tunneling current is kept constant by varying the tip-surface separation. The $z$-piezo regulation voltage is recorded and converted to a height. In (b) the tunneling current is measured at a constant height. The tunneling current depends on the tip-surface separation.

Figure 2

Three $83\times 14{\mathrm{nm}}^2$ high-speed STM images at $130\mathrm{K}$, showing a step on the Cu(1 1 17) surface partly covered with indium atoms. Indium atoms that are deposited on a Cu(001) surface are embedded in the first layer of the surface. At room temperature the indium atoms are mostly incorporated through steps. Between (a) and (b) two of the indium rows [encircled in image (a)], which decorate the kinks along the step, are exchanging indium atoms. (c) The situation where several indium atoms have moved from the leftmost of the two rows to the right row. Time per image is $0.64\mathrm{s}$, with sample bias of $-2.083\mathrm{V}$, and tunneling current of $0.1\mathrm{nA}$. From 90.

Figure 3

(Color online) Schematic representation of the atom-tracking mode. The tip is dithered above the adsorbed atom in (a) and the lateral feedback [shown schematically in (b)] responds to the local slope, forcing the tip to climb uphill. For example, when the tip is offset to the left, the slope is positive and the tip is pushed back to the right.

Figure 4

(Color online) STM images $(4\times 3{\mathrm{nm}}^2)$ of a Si(001) surface with a Si ad-dimer with its dimer bond aligned (a) parallel or (b) perpendicular to the dimer bonds of the underlying substrate dimer row. In (c) the measured $z$ signal is shown as a function of time. The transitions between A and B positions show up as sharp changes in the measured $z$ signal. From 86.

Figure 5

(Color online) Adsorption and dynamics of propene on Cu(211). (a) STM image $(6.1\times 4.9{\mathrm{nm}}^2)$ recorded at $7\mathrm{K}$ of propene on Cu(211). The intrinsically stepped surface appears as dark and bright stripes. The bright stripes are located near the step edges. Two species of adsorbates (1,2) are observed both appearing in two mirror-related states (1, $1^{*}$ and 2, $2^{*}$, respectively). (b) Lowest energy adsorption of states 1 and $1^{*}$. (c) Lowest energy adsorption of states 2 and $2^{*}$. From 59.

Figure 6

(Color online) Adsorption and dynamics of acetylene on Cu(100). (a) Schematic of acetylene on Cu(100) showing side and top views of the molecular adsorption site and orientations consistent with the STM images. The dashed line represents the outline of the dumbbell-shaped depression in STM images. The asterisk refers to the position of the STM tip. The images are scanned at a tunneling current of $10\mathrm{nA}$ and a sample bias of $100\mathrm{mV}$. The square lattice represents the position of the atoms of the Cu(100) surface. (b) Current during a $364\mathrm{mV}$ voltage pulse over an acetylene molecule initially in the high-current orientation, while the STM tip remains fixed $0.15\mathrm{nm}$ off center. Each jump in the current indicates the moment of rotation of the molecule. (c) Distribution of the times the molecule spent in the high-current orientation with a fit to an exponential decay with a time constant of $184\mathrm{ms}$. From 79.

Figure 7

Ball and stick models of the silicon and germanium (001) surfaces: (a) $1\times 1$ (unreconstructed) surface, (b) $p(2\times 1)$ dimer reconstruction, (c) $c(4\times 2)$ dimer reconstruction, and (d) $p(2\times 2)$ dimer reconstruction. The surface unit cells are outlined.

Figure 8

(Color online) Schematic diagram of a diffusing phason. The phason diffuses from a distance $3a$ from the origin in (a) to a distance $5a$ from the origin in (c). One diffusion “step” of a phason corresponds to a single flip-flop event of one of the constituting dimers of the phason.

Figure 9

(Color online) Filled-state room-temperature STM image of Ge(001). The sample bias is $-1.5\mathrm{V}$ and the tunneling current is $0.4\mathrm{nA}$. The boundaries between the local $c(4\times 2)$ and $(2\times 1)$ reconstructions are indicated by white lines. The inset shows a schematic representation of a buckled dimer. The tilt angle of the dimer bond is 10°–20°.

Figure 10

(Color online) Dynamics of Ge dimers in the proximity of missing dimer defects. (a) Filled-state STM image of Ge(001). The sample bias is $-1.5\mathrm{V}$ and the tunneling current is $0.4\mathrm{nA}$. The dimers appear fuzzy in some of the substrate dimer rows. This flickering is a result of the flip-flop motion of the dimers during imaging. The flickering is most pronounced in dimer rows that contain missing dimer defects. Note that this flickering occurs in a symmetric dimer row (right defect) as well as in an asymmetric dimer row (left defect). Labels 1–4 refer to the different types of dimers [a flickering asymmetric dimer (1), a flickering symmetric dimer (2), a nonflickering symmetric dimer (3), and a nonflickering asymmetric dimer (4)] over which the tunneling current is measured as a function of time. (b) Some of the dimer positions where the current is measured as a function of time are labeled A–F. Note that (b) is rotated with respect to (a).

Figure 11

(Color online) Current traces measured on the dimers of a Ge(001) surface. The current is recorded over a flickering asymmetric dimer [curve (1)], a flickering symmetric dimer [curve (2)], a nonflickering symmetric dimer [curve (3)], and a nonflickering asymmetric dimer [curve (4)]. The sampling rate is $50\mathrm{kHz}$. Current set points are $0.40\mathrm{nA}$ for traces (1)–(3) and $0.55\mathrm{nA}$ for trace (4).

Figure 12

(Color online) Histogram of the residence times in the two buckled states of a symmetric appearing flickering dimer from Fig. 10a. The gray line is the theoretical fit for a random process (the Poisson distribution). $\tau \left(1\right)$ and $\tau \left(2\right)$ are the counts for the residence times in the two buckled states.

Figure 13

(Color online) Statistics of the dynamics of dimers of Ge(001). (a)–(d) Histograms of the residence times for dimers A–D from Fig. 10b. The lines are the corresponding theoretical curves for a random process. $\tau \left(1\right)$ and $\tau \left(2\right)$ are the counts for the residence times in each of the two buckled states.

Figure 14

(Color online) STM images revealing thermal activation of thioether rotors on Au(111): dimethyl, diethyl, and dihexyl sulfide. The temperatures in the right column are the onset temperatures for the rotation process. The sample bias was $-300\mathrm{mV}$ and the tunneling current was $9\mathrm{pA}$. From 2.

Figure 15

(Color online) Tunneling current vs the time recorded over a dibutyl sulfide molecule on a Au(111) surface. Sudden jumps in the current indicate changes in the position of the alkyl tail of the thioether with respect to the STM tip. Three current levels correspond to the three inequivalent orientations of the dibutyl sulfide molecule with respect to the position of the STM tip (indicated by the black dot). From 2.

Figure 16

(Color online) Dynamics of Pt atomic chains on Ge(001). (a) An STM topograph of atomic chains on Ge(001) at $4.7\mathrm{K}$. Image size is $65\times 65{\mathrm{\AA }}^2$, bias voltage is $-1.5\mathrm{V}$, and tunneling current is $0.5\mathrm{nA}$. The atomic chains show up as bright protrusions running from the bottom to the top in the image. (b) Top view of a regular dimer pair at $77\mathrm{K}$, bias voltage of $-1.0\mathrm{V}$, and tunneling current of $0.8\mathrm{nA}$. The atomic chain runs from left to right in this image. (c) and (d) Two subsequent images of a dimer pair that exhibits dynamics. The atomic chains run from left to right in these images. The reconfiguration of the dimer pair takes place on a time scale which is far below the time needed to conduct one STM image and shows up as a discontinuity as the tip is scanned across the chain. (Note that the fast scanning direction is from top to bottom, the slow scanning direction is from left to right in this images.)

Figure 17

(Color online) Statistics of the dynamics of Pt atomic chains on Ge(001). (a) The measured flip-flop frequency of the dimer pair as a function of the tunneling current. The frequency depends linearly on the tunnel current and passes through the origin (see the least-squares fit indicated by the dotted line). Each data point plotted is the average of 100 values. (b) Current traces, showing telegraphic signals, resulting from different dimer-pair flipping modes at $77\mathrm{K}$ (measured with open feedback loop, bias voltage of $-1.0\mathrm{V}$, and set-point current of $1.0\mathrm{nA}$). The graph is corrected for slow variations in the tunnel current as a result from drift of the STM tip. The dimer pair switches between six well-defined states, indicated by the dotted lines in the graph.

Figure 18

(Color online) $I\left(t\right)$ measurements of the dynamic dimer pair at $77\mathrm{K}$ with open feedback loop and a bias voltage of $-1.0\mathrm{V}$. The set points of the tunnel current are (a) 1.5, (b) 0.8, (c) 0.8, and (d) $1.5\mathrm{nA}$. The dynamic dimer pair flips back and forth between two well-defined current levels. The decrease of the tunnel current with time in (a) results from the drift of the STM tip. Panels (b)–(d) are corrected for this drift. (b) Three well-defined current levels, of which one is common in both observed flipping modes. (c) and (d) Four current levels, with two different transitions between the two flipping modes. The different flipping modes are shown in Fig. 19b.

Figure 19

(Color online) Schematic overview of the various configurations of the flipping dimer pair. (a) Measured boundary positions of a flipping dimer pair extracted from STM topographs. The atoms of the dimer pair are marked as pivot atoms $\left(p\right)$ or revolving atoms $\left(r\right)$. The motion of the dimers is indicated by the gray arrows. The thick dashed lines indicate a down-down, down-up, or a up-up configuration. The motion of the dimer pair resembles the flippers of an atomic pinball machine. (b) Schematic of the flipping dimer pair. Each flipper is in either a down or an up configuration, resulting in four possible configurations. The pivot and revolving atoms are shown. Colors refer to the flipping modes highlighted in (a). The tip position is indicated with the triangle. (c) The four possible configurations in (b) can lead to six different flipping modes. (In mode 1, for instance, the first dimer flips up and down, while the second dimer remains in the up configuration.) Flipping mode 5 has never been observed experimentally. (d) Including an attractive interaction between the two revolving atoms of the flipping dimer pair leads to a smaller amplitude of oscillation as compared to the case where only one of the two flippers flips up and down. This shows up as an additional flipping mode with two corresponding additional current levels [dotted lines in (c)].