Hawkes P.W., Spence J.C.H. (Eds.) Science of Microscopy. V.1 and 2

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1110 U. Weierstall

0.37 nm above the surface compared to 0.7 nm for the gas phase mole-

cule. Upon adsorption at room temperature the molecules diffuse to

step edges and are stabilized with their wire parallel to step edges,

which prevents good electronic contact (Kuntze et al., 2002).

The problem of controlling the electronic contact between the mole-

cule and its electrodes has been addressed in an investigation with the

Lander molecule. Reproducible contact formation was obtained by

lateral manipulation of the Lander on Cu(111) with an LT-STM (Moresco

et al., 2003). The molecules where adsorbed at 70 K instead of room

temperature to avoid postdeposit thermal diffusion. Figure 17–34 (A3)

shows an STM image of a single Lander on a terrace where the four

bumps are attributed to the four legs of the molecule. When a molecule

is pushed to the step with its central wire parallel to it, it reaches a ﬁ nal

conformation imaged in Figure 17–34 (B3). Separated by the legs, the

central wire is not interacting with the step edge and the standing wave

pattern (LDOS oscillations) (B4) on the upper terrace has not changed.

If the molecule is repositioned with its wire oriented perpendicular to

the step edge, a notable modiﬁ cation of the standing wave pattern on

the upper terrace is observed. The amplitude of the standing wave

pattern is reduced at the contact point compared to the clean step edge

case (4C). Simulations of the standing wave pattern indicated that the

perturbation is caused by the terminal part of the molecular wire on

the upper terrace. This contact area is visible in the STM image as an

additional small bump (C3) and its location was conﬁ rmed by elastic-

scattering quantum chemistry STM image calculations (Sautet and

Joachim, 1991) (C2). The molecule could also be decontacted by reverse

lateral manipulation, and the original step edge and molecule image

were recovered.

Figure 17–33. Top view of the chemical structure of the Lander molecule,

which was designed to be a model system for a molecular wire. The molecular

wire and the four legs that support the wire are shown. (From Moresco et al.,

2003.)

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1111

Figure 17–34. Contacting a molecular wire to a step: Lander molecules on a

step-free Cu(111) surface (column A) and contacted to a (100) step. Molecular

wire parallel (column B) and orthogonal (column C) to the step. Row 1: Sphere

models of molecular structures. Row 2: Calculated STM images corresponding

to the sphere models above. Row 3: STM images at 8 K. Row 4: STM measure-

ments showing LDOS oscillations. In (C2) and (C3), an additional bump

appears corresponding to the contact point of the wire to the step. Modiﬁ ca-

tion of the standing wave pattern on the upper terrace is observed only when

the wire is orthogonal to the step (C4). (From Moresco et al., 2003.)

1112 U. Weierstall

In a variable-temperature STM experiment it has been found that the

Lander molecule can act as a template, self-fabricating short metallic nano-

structures at step edges. Lander molecules where adsorbed at room tem-

perature on the Cu(110) surface and their conformation and anchoring at

step edges were studied at 100 K. At room temperature the Cu kink atoms

are highly mobile and the Lander molecule reshapes the ﬂ uctuating Cu

step adatoms into tooth-like nanostructures perpendicular to the step

edges. The dimension and shape of the Lander molecule form a perfect

template for a double row of Cu atoms. Moving the molecule away form

the step edge by lateral manipulation at low temperature revealed the

underlying restructuring of the step edges (Figure 17–35). Upon adsorp-

tion of the Lander molecules at 150 K, no restructuring of the Cu step edges

was observed, since the mobility of the Cu kink atoms at this temperature

is not high enough for the molecular template to be effective.

At low temperatures, the short Cu nanowire acts as a sliding “rail”

for the Lander molecule. By moving the molecule to the end of such a

nanostructure, a model geometry can be obtained where one end of

the central molecular wire is electronically connected to the metallic

wire. A detailed study of the lateral manipulation of the Lander mole-

cule along such an atomic wire has been performed with an LT-STM

at 8 K (Grill et al., 2004). Lateral displacements of the molecule have

been separated into monoatomic steps and it has been shown that

single molecular legs can be rotated reversibly while keeping the

Figure 17–35. Self-assembly of nanowires at step edges initiated by Lander molecule adsorption. (A–

D) Low-temperature STM images of a manipulation sequence of the Lander molecules from a step

edge on Cu(110) The arrows show which molecule is being pushed aside; the circles mark the tooth-

like structures that are visible on the step where the molecule was docked. (E) Zoom-in STM image

showing the characteristic two-row width of the tooth-like structure after removal of a single Lander

molecule. (From Rosei et al., 2002.)

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1113

central wire ﬁ xed. Comparison with theory conﬁ rmed that the central

wire is contacted to the metallic Cu atomic wire.

In a recent LT-STM study of the Lander molecule, it has been shown

that despite the fact that the legs elevate the molecular wire away from

the surface, there is still an electronic interaction between the central

wire and the surface states of the substrate (Gross et al., 2004). This

was shown by comparing the standing wave patterns of surface-state

electrons scattered off the molecule with calculated patterns taking

into account scattering from different areas of the molecule.

Chemical bond formation was studied with an LT-STM (Lee and Ho,

1999). Individual iron atoms were evaporated and coadsorbed with CO

molecules on an Ag(110) surface at 13 K. A CO molecule was trans-

ferred from the surface to the STM tip and bonded with an Fe atom on

the surface to form Fe(CO). A second CO molecule could then be added

to form Fe(Co)

. This is shown in Figure 17–36. The adsorption sites of

Figure 17–36. Bond formation induced with STM tip. A sequence of STM

constant-current images at 13 K showing the formation of Fe–CO bonds by

vertical manipulation. Fe atoms are imaged as protrusions and CO molecules

as depressions. The white arrows indicate the pair of adsorbates involved in

each bond formation step. In (B) and (C) a CO molecule has been picked up

and bonded to an Fe atom to form Fe(CO). In (D) a second CO molecule has

been bonded to Fe(CO) to form Fe(CO)

. (From Lee and Ho, 1999.) (See color

plate.)

1114 U. Weierstall

Fermi level

Metal A Metal B

Figure 17–37. Elastic and inelastic tunneling channels. Tunneling electrons

can excite a molecular vibration of energy

hω only if eU >

hω. For smaller

energies, there is no ﬁ nal state into which the electron can tunnel. Therefore

the inelastic current has a threshold at

hω/e. The increase in conductance at

the threshold is typically 1–10% in an STM experiment.

the reactants could be determined by resolving the underlying Ag

lattice with a CO molecule attached to the tip, which leads to increased

resolution in the constant-current image. This increase in spatial reso-

lution can be attributed to the more localized wavefunction of the

molecule-terminated tip. Analysis with inelastic tunneling spectros-

copy provided spectroscopic support for the identiﬁ cation of the created

single molecule products with Fe(CO) and Fe(CO)

Assembly of an artiﬁ cial nanostructure composed of a copper(II)

phthalocyanine (CuPc) molecule bonded to two gold atomic chains on

NiAl(110) has been realized with an LT-STM (Nazin et al., 2003b). The

electronic structure of this model metal–molecule–metal junction was

studied by spatially resolved STS and systematically tuned by varying

the number of gold atoms in the chains. Splitting and shifting of

molecular orbital energies and modiﬁ cation of the local electronic

structure of the electrodes were observed. These effects determine the

alignment of the molecular orbital energies with respect to the Fermi

energy of the metal and affect the conductivity of the junction.

3.3 Local Inelastic Electron Tunneling Spectroscopy

Besides the dominant elastic electron tunneling process, for which the

electron energy is equal in the initial and ﬁ nal state, inelastic tunneling

can occur if the tunneling electrons couple to some modes ω in the

tunneling junction. Figure 17–37 shows an energy diagram for T = 0,

illustrating elastic and inelastic tunneling processes. In the case of

inelastic tunneling the electron loses energy

hω to a mode in the tun-

neling barrier. According to the Pauli exclusion principle, tunneling is

possible only if the ﬁ nal state after the inelastic tunneling event is ini-

tially unoccupied. The bias voltage dependence of the tunneling current

with inelastic tunneling is shown schematically in Figure 17–38. The

elastic tunneling current increases linearly, proportional to V. As long

as the bias voltage is smaller than the lowest energy mode that can be

excited in the gap, inelastic tunneling processes cannot occur. At the

threshold bias V =

hω/e, the inelastic channel opens up, and the number

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1115

of electrons using this channel increases linearly with V. Therefore the

total current has a kink at the threshold bias voltage. In the differential

conductance curve dI/dV, the kink becomes a step and the second

derivative d

I/dV

exhibits a peak at the threshold. If several modes ω

can be excited in the tunneling process, each mode leads to a peak in

the differential conductance at the corresponding voltage V

hω

/e.

Inelastic electron tunneling spectroscopy (IETS) can therefore be

regarded as a special form of electron energy loss spectroscopy.

total

elastic

dI/dV

I/dV

Figure 17–38. Sche-

matic current versus

voltage curves with

elastic and inelastic

tunneling. A kink is

observed when the

inelastic tunneling

channel opens up. The

kink becomes a step in

the ﬁ rst derivative and

a peak in the second

derivative.

1116 U. Weierstall

IETS has been shown to be a powerful technique for measuring the

vibrational spectra of molecules that have been intentionally incorpo-

rated into a metal–oxide–metal tunneling junction (Jaklevic and Lambe,

1966). Vibrational spectroscopy can be performed with a variety of

other techniques including electron energy loss spectroscopy, infrared

absorption spectroscopy, Raman spectroscopy, inelastic neutron scat-

tering, and helium atom scattering. All of these techniques have in

common with IETS that they rely on macroscopic numbers of mole-

cules to achieve detectable signal levels. The signal is therefore an

average over molecules whose local environment can vary. The major

drawback of traditional IETS with planar metal–oxide–metal junctions

is that the molecules are buried within the junction, which is difﬁ cult

to characterize microscopically.

Replacing the oxide layer by vacuum and the top planar electrode

by a sharp STM metal tip has made it possible to extend IETS to single

adsorbed molecules. One great advantage of performing vibrational

spectroscopy with the STM is that the high spatial resolution of STM

images permits changes in molecular spectra to be correlated with

variations in the local environment on an atomic scale. STM-IETS was

proposed as early as 1985 (Binnig et al., 1985b). Since the changes in

tunneling conductance resulting from opening of additional inelastic

tunneling channels are typically 0.1–1% for planar junctions and 1–10%

for STM junctions, the relative stability of the tunneling current has to

be better than 1% to obtain reasonable IET spectra with the STM. The

physics of tunneling then dictates a tunneling gap stability of better

than ∼0.005 Å

over the time it takes to complete one scan of the spec-

trum (Lauhon and Ho, 2001). Because the vibrational features are very

sharp, liquid helium temperatures are required to avoid thermal broad-

ening of the Fermi levels. Hansma (1982) estimated an effective resolu-

tion of 5.4k

T (∼140 mV at room temperature) for inelastic tunneling,

while vibrational features are typically only a few millivolts wide. For

those reasons, vibrational spectroscopy with the STM has proved dif-

ﬁ cult. First experiments probing a cluster of sorbic acid molecules

adsorbed on graphite at 4 K reported large jumps in the ﬁ rst derivative

spectrum instead of the expected second derivative spectrum (Smith

et al., 1987). The peaks where attributed to characteristic vibrations of

molecules. However, due to molecular diffusion events during the

measurements, the spectra were not very reproducible and the energies

of the peaks were different form those measured in bulk tunnel junc-

tions. Reproducible single-molecule vibrational spectroscopy has been

achieved only recently with an LT-STM (Stipe et al., 1998). In these

landmark experiments, a Cu(100) surface was dosed with acetylene

) and deuterated acetylene (C

). Vibrational spectra where

aquired at 8 K above single molecules with the use of a tracking scheme

to position the tip at the center of the molecule, with lateral and vertical

resolution of better than 0.1 and 0.01 Å

, respectively. Contributions

from the electronic spectrum of the tip and the substrate could be

minimized by subtracting spectra taken over a clean area of the surface

from the molecular spectra. The I–V curves from a single molecule and

the clean surface (Figure 17–39A) show the expected linear dependence

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1117

for metallic junctions. The differential conductance dI/dV shows an

increase of 4.2% at 358 mV, resulting from the excitation of the C—H

stretch mode (Figure 17–39B). The second derivative d

I/dV

reveals a

distinct peak at 358 mV (Figure 17–39C, compare with the idealized

view of Figure 17–38). An isotopic shift to 266 mV was observed for

deuterated acetylene and the C—D stretch mode. These values are in

close agreement with those obtained by EELS for the same molecules

on Cu(100). The ability to spectroscopically identify molecules with the

STM makes it possible to implement chemical-sensitive STM imaging.

This has been demonstrated by recording a d

I/dV

map above both

acetylene isotopes. When the dc voltage was ﬁ xed at 358 mV, only one

of the two molecules (C

) was imaged, whereas at 266 mV, the other

molecule (C

) was imaged (Figure 17–40). Thus, individual adsorbed

molecules can be identiﬁ ed by their vibrational spectra and inelastic

images. In contrast, identiﬁ cation and characterization of adatoms and

Figure 17–39. Molecular vibrational spectra observed with the STM at 8 K. (A)

I–V curves recorded with the STM tip directly over the center of an acetylene

molecule (1) and over the bar Cu(100) surface (2). (8) dI/dV on the molecule

(1) and on the substrate (2). (C) d

I/dV

on the molecule (1) and on the substrate

(2). A peak at 358 mV is visible in the difference spectrum. (3) An average over

279 scans of 2 min each (10 h total data acquisition time directly above the

molecule) with a different tip. The conductance change due to inelastic tun-

neling was 3–4% with different tips. (From Stipe et al., 1998.)

1118 U. Weierstall

molecules by electronic spectroscopy with the STM are problematic

because the electronic energy levels are broadened and shifted upon

adsorption and the adsorbed molecule’s spectrum becomes convolved

with the STM tip’s electronic spectrum (Crommie et al., 1993c).

STM-IETS has been used to determine the orientation of individual

HD molecules (deuterated acetylene) adsorbed on the Cu(100) surface

at 8 K (Stipe et al., 1999a). By setting the bias voltage to the vibrational

energy of the C—D stretch mode in C

HD, and simultaneous recording

the constant-current image and the vibrational image (d

I/dV

map),

the spatial distribution of the C—D stretch signal within the molecule

could be determined. Since the inelastic image has its maximum near

the midpoint of the C—D bond, it locates the position of the bond in

this case.

The previous two examples show the ability of the LT-STM to resolve

internal vibrations of molecules adsorbed on surfaces. The internal

modes can be used in surface chemical analysis for the identiﬁ cation

of adsorbed species. External vibrations, i.e., vibrations of adsorbed

molecules with respect to the substrate, are more sensitive to the inter-

action of the adsorbed molecule with the substrate. These external

modes generally have lower energy than the internal ones, and are not

easily accessed by some of the averaging vibrational spectroscopy

methods mentioned above. External vibrational modes of benzene

molecules on an Ag(110) surface have been detected with an LT-STM

at 4 K (Pascual et al., 2001a). These measurements conﬁ rmed that the

Figure 17–40. Chemical sensitive imaging. Spectroscopic spatial images of the

inelastic channels for C

and C

. (A) Constant-current image of a C

(left) and a C

(right) molecule. This image is an average of STM images

recorded simultaneously with the vibrational images. The molecules appear

identical in this normal imaging mode. d

I/dV

maps (vibrational images) of

the same area were recorded at (B) 358 mV, (C) 266 mV, and (D) 311 mV. In (B)

only C

is visible, whereas in (C) C

is visible. The symmetric, round

appearance of the molecules is attributed to the rotation of the molecule

between two equivalent orientations during the experiment. (From Stipe et al.,

1998.)

Chapter 17 Low-Temperature Scanning Tunneling Microscopy 1119

external vibrations are strongly sensitive to the nature of the molecule–

substrate bond. For internal vibration modes, the inelastic signal has

been shown to be very localized at the position of the particular bond

excited (Stipe et al., 1999a). In contrast the spatial distribution of the

inelastic tunneling, signal for external modes extends over the whole

area of the molecule (Pascual et al., 2001a).

In a detailed study of temperature effects, electronic structure

contributions, and tip effects on STM-IETS, Lauhon and Ho (2001)

suggested that functionalization of the tip by transfer of a known mol-

ecule to the tip offers a means of accessing different vibrational modes.

This has been conﬁ rmed in later experiments (Moresco et al., 1999;

Hahn and Ho, 2001) where CO and C

molecules have been trans-

ferred to the tip and single CO and O

molecules were probed with

this tip.

There are a few caveats regarding the application and interpretation

of STM-IETS spectra. In STM-IETS as in traditional IETS, there are no

strict selection rules. Modes involving motion parallel and perpendicu-

lar to the surface can be excited. Modes are not observed for all mole-

cules and not all modes are necessarily observed for any particular

molecule. The symmetry of vibrational modes and electronic reso-

nances of an adsorbate seem to give rise to selection rules for vibra-

tional mode detection in STM-IETS (Lorente et al., 2001). A dependence

of the STM-IETS signal on the molecular orientation on the surface has

been shown for C

on Ag(110) (Pascual et al., 2002). The spectroscopic

maps showed a correlation between the enhanced vibrational signal

and orientational symmetry of the adsorbed molecule observed in

constant-current mode.

To complicate matters more, it has been shown that vibrational exci-

tation can lead to suppression of elastic tunneling and produce dips

instead of peaks in the differential conductance spectrum (Hahn et al.,

2000). This has been recently explained theoretically by Lorente (2004).

Despite this complexity, there are many advantages of STM-IETS, i.e.,

that the adsorbate geometry is well deﬁ ned and the effect of adsorbate

orientation can be studied systematically. Recent progress in the theo-

retical analysis of STM-IETS may greatly enhance its ability to probe

chemistry at the spatial limit (Mingo and Makoshi, 2000; Makoshi and

Mingo, 2002; Lorente and Persson, 2000; Lorente, 2004).

3.4 STM-Induced Photon Emission

Injection of electrons or holes form the tip of an STM into the surface

leads to the emission of light for many materials. The ﬁ rst observation

of light emitted from the tunneling junction of an STM in the low-bias

tunneling regime (eV < Φ where Φ is the work function) goes back to

Coombs et al. (1988). The highly localized tunneling current allows

high spatial resolution that enables experiments with single nanostruc-

tures and molecules. Photon emission from the tunneling junction of

an STM can be used to measure the optical properties of the sample

surface in the nanometer regime. Photon emission from metals involves

surface plasmons, which are inelastically excited by the tunneling elec-