Formation of Co Atomic Wire in Hydrogen Atmosphere - American

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Formation of Co Atomic Wire in Hydrogen Atmosphere Tomoka Nakazumi† and Manabu Kiguchi*,†,‡ †

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan, and ‡PRESTO, Japan Science and Technology Agency, 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

ABSTRACT We have investigated the conductance and atomic structure of the Co atomic contact in a hydrogen atmosphere. While clean Co did not form an atomic wire, the Co atomic contact could be stretched more than 0.4 nm in a hydrogen atmosphere, indicating the formation of the atomic wire. The interaction between hydrogen and the Co atomic wire was investigated with inelastic tunneling spectroscopy (IETS). The vibrational modes between hydrogen and the Co atomic wire were observed in IETS. The length histogram of the last conductance plateau, the shape of the dI/dV curves, the distribution of the vibrational energy, and previously reported theoretical calculation results suggested the formation of the Co atomic wire in which dissociated hydrogen atoms were adsorbed on the wire. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

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Recently, Thijssen et al. reported the formation of the Ag (4d metal) atomic wire in an oxygen atmosphere. They discussed the formation of the Ag atomic wire based on the surface reconstruction.11,12 It has been argued that the underlying physical mechanism for the formation of atomic wires of 5d metals is the same as that for the surface reconstructions on clean surfaces of these 5d metals. While the clean Ag surface does not reconstruct, the Ag surface reconstructs when oxygen adsorbs on the Ag surface. The Ag atomic wire could be, thus, formed in an oxygen atmosphere. Atomic wires of other 3d and 4d metals could be formed in the presence of molecules. Among ferromagnetic metals, we have paid attention to Co. Since the Co surface reconstructs when hydrogen adsorbs on the surface,13 the Co atomic wire could be formed in a hydrogen atmosphere. The conductance behavior of the Co atomic contacts in hydrogen atmosphere was investigated by Untiedt et al.14 The conductance histogram showed a feature at around 1 G0 after admitting hydrogen. However, the origin of this feature is not clear up to now. In the present study, we have investigated the formation of the Co atomic wire in a hydrogen environment. The interaction between hydrogen and the Co atomic wire was investigated by using inelastic electron tunneling spectroscopy (IETS).9,15 IETS provided the information of the vibrational modes between Co atoms in the Co atomic wire and the mode between hydrogen and the Co atomic wire. Figure 1 shows the conductance trace and conductance histogram of the Co nanocontacts before and after admitting hydrogen. The stretch length was the displacement of the distance between the stem parts of the Co electrodes, which were fixed with epoxy adhesive. Before admitting hydrogen,

onoatomic wires of ferromagnetic metals have attracted wide attention for fundamental science and technological applications.1 Various interesting phenomena, such as ferromagnetic transition and spindependent electron transport, have been predicted for the atomic wires of ferromagnetic metals by theoretical calculation.2,3 The enhancement of the localized orbital moment and large magnetic anisotropy were experimentally observed for the Co atomic wires on a Pt step surface.4 Phase transition from ferromagnetic to superparamagnetic was observed for the Fe atomic wires on a Au surface.5 While various interesting phenomena have been observed for the atomic wires of ferromagnetic metals on the substrate, the interaction between the metal atomic wire and the substrate cannot be negligible. It is strongly desired to fabricate and investigate the properties of the atomic wires of ferromagnetic metals which do not interact with the substrate. A metal atomic wire bridging between two metal electrodes is one of the appropriate atomic wires to investigate the metal atomic wire itself. Metal atomic wires bridging between metal electrodes have been fabricated with scanning tunneling microscope (STM), mechanically controllable break junction (MCBJ), and other techniques.1 The atomic image of the Au atomic wire was reported using a transmission electron microscope (TEM).6 The formation of the atomic wires was reported for the 5d metals (Au, Pt, and Ir) by the conductance measurement in ultrahigh vacuum (UHV) at 4 K.7-9 On the other hand, the fabrication of the atomic wires of ferromagnetic metals (3d and 4d metal) is difficult even in UHV at low temperature due to their mechanical instability. The atomic wire can be formed if the binding energy per bond is much larger in the atomic wire than that in the bulk. The atomic wires of 5d metals can meet this requirement due to the relativistic effects.10 On the other hand, the atomic wires of 3d and 4d metals could not meet this requirement.

r 2010 American Chemical Society

Received Date: January 21, 2010 Accepted Date: February 11, 2010 Published on Web Date: February 22, 2010

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DOI: 10.1021/jz100084a |J. Phys. Chem. Lett. 2010, 1, 923–926

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reported one.14 Atomic contacts with conductance values of 1.0 and 0.2 G0 were preferentially formed while stretching the Co contact in a hydrogen atmosphere. In addition to the change in the conductance value of the Co atomic contact, the increase in the length of the conductance plateau was observed for the Co atomic contact after admitting hydrogen. The conductance trace in Figure 1a showed that the Co atomic contact could be stretched to quite long lengths (>0.5 nm), which suggested the formation of an atomic wire. Similar elongation of the conductance plateau was also observed for the Ni atomic contact after admitting hydrogen. In order to investigate the wire formation, we measured the length histogram of the last conductance plateau and the return length distribution. Figure 2 shows the length histogram of the last conductance plateau for the Co contacts before and after admitting hydrogen. The length for the Co contacts in a hydrogen atmosphere was taken here as the distance between the points at which the conductance dropped below 2.2 and 0.1 G0, while for the clean Co contacts, the boundaries were 2.2 and 1.2 G0. For the clean Co contact, the contact broke within 0.2 nm. Since the Co-Co distance is 0.25 nm for bulk Co, the short plateau length indicated that Co did not form an atomic wire. In a hydrogen atmosphere, the Co contact could be stretched up to 0.5 nm, which corresponded to the monoatomic wire of about two Co atoms. Hydrogen would adsorb on the surface or be incorporated into the atomic wire in a hydrogen atmosphere, and thus, the stable Co atomic wire could be formed in a hydrogen atmosphere. The formation of the Co atomic wire was supported by the return length distribution for the Co atomic wire in a hydrogen atmosphere. The inset of Figure 2 shows the average return lengths as a function of plateau length. The return length was the distance over which the two electrodes needed to be moved back after the junction broke in order to reestablish contact, averaged over many break cycles. Apart from an offset of 0.06 nm due to the elastic response of the banks, the relation was approximately proportional. The obtained result suggested that a fragile structure was formed with a length corresponding to that of the last plateau, which was unable to support itself when it broke and collapsed onto the banks on either side.1,7 Next, IETS was measured for the Co atomic wire in a hydrogen atmosphere in order to investigate the interaction

the conductance decreased in a stepwise fashion, and the corresponding conductance histogram showed a peak at around 1.4 G0, which corresponded to the clean Co atomic contact.1,14 After admitting hydrogen, the conductance decreased in a stepwise fashion below 1.0 G0, and the corresponding conductance histogram showed peaks at around 1.0 and 0.2 G0, accompanied by a low conductance tail. The obtained conductance histogram agreed with the previously

Figure 1. (a) Breaking and return conductance traces for the clean Co contacts (black line) and for the Co contacts in a hydrogen atmosphere (red line). (b) Conductance histograms for the clean Co contacts (black line) and for the Co contacts in a hydrogen atmosphere (red line). Each conductance histogram was constructed from 1000 conductance traces recorded with a bias of 0.1 V during breaking of the contact.

Figure 2. Length histogram for clean Co contacts (black line) and for Co contacts in a hydrogen atmosphere (red line). Inset: Average return lengths as a function of plateau length for the Co contacts in a hydrogen atmosphere.

Figure 3. Example of dI/dV and d2I/dV2 curves for the Co atomic wires in a hydrogen atmosphere. The peaks were observed at (a) 49, (b) 43, and (c) 46 meV in d2I/dV2 curves.


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the number of the conductance channel was more than one. When the hydrogen bridges between Co electrodes, the electron will be transmitted through the hydrogen 1s atomic orbital (in the case of a hydrogen atom bridge) or bonding or antibonding molecular orbitals (in the case of hydrogen molecule bridge), which would be modulated by Co.19 Since these orbitals are not degenerate and energetically wellseparated, only one orbital would contribute to the electron transport through the hydrogen atom or molecule bridge. Since the present experimental results suggested that the number of the conduction channel was more than one, hydrogen would not bridge between the Co electrodes. The electron would be transmitted through Co d orbitals, which would be modulated by the adsorbed hydrogen. Finally, we discuss whether the hydrogen atom or molecule adsorbed on the Co atomic wire. The previous theoretical investigation reported the vibrational modes for the Pt contact in which hydrogen atoms adsorbed on the contact (the structure model is like the inset of Figure 4).20 Three vibrational modes were observed at 31, 42, and 68 meV below 80 meV. The 31, 42, and 68 meV corresponded to the vibrational mode in which two hydrogen atoms moved in the same direction perpendicular to the stretching axis, moved in the opposite direction perpendicular to the stretching axis, and moved in the opposite direction along the stretching axis, respectively. The previous experimental and theoretical investigation revealed that the interaction between the hydrogen atom and Pt surface was close to the Co surface (cf. the hydrogen-metal binding energies of 2.64 eV for Pt and 2.59 eV for Co).21-25 Similar vibrational modes would be observed for the Co atomic wire with hydrogen atoms. On the basis of the previously reported experimental and theoretical calculation, the vibrational mode at around 15 meV observed in the present study could be assigned to the vibrational mode of Co metals. The vibrational mode at around 30 meV observed in the present study could be assigned to the superposition of the vibrational mode of Co metals and the vibrational mode in which two hydrogen atoms moved in the same direction perpendicular to the stretching axis. The vibrational mode at around 48 meV could be assigned to the vibrational mode in which two hydrogen atoms moved in the opposite direction perpendicular to the stretching axis. In conclusion, we have investigated the formation of the Co atomic wire in a hydrogen atmosphere by conductance measurement and IETS. After admitting hydrogen, the conductance histogram showed a feature at around 1.0 G0, and the length histogram of the atomic wire showed that the Co atomic contact could be stretched more than 0.4 nm. The 0.4 nm indicated the atomic wire of two Co atoms. The formation of the Co atomic wire was supported by the return length distribution. The IETS showed three peaks at around 15, 30, and 48 meV, which could be assigned to the vibrational modes of Co atoms, the superposition of the vibrational mode of Co atoms and the vibrational mode between hydrogen and the Co atomic wire, and the vibrational mode between hydrogen and Co atomic wire, respectively. The Co atomic wire could be stabilized due to the strong interaction between hydrogen and the Co atomic wire.

Figure 4. Distribution of the vibrational energy for the Co atomic wire in a hydrogen atmosphere. The red curve is the least-squares fit. The inset shows the structural model.

between hydrogen and the Co atomic wire. Figure 3 shows the example of differential conductance and its derivative (IETS) as a function of the bias voltage for the Co atomic wire taken at a conductance of around 0.1-1.2 G0. A symmetric upward step was observed in differential conductance at around 10-60 meV, and clear symmetric peaks were observed in its derivative. The conductance enhancement is explained by the opening of an additional tunneling channel for electrons that lost energy to a vibrational mode.1,9 This conductance enhancement was typical for the IETS. Here, it should be noticed that the conductance enhancement was observed even for the contact showing conductance of around 1.2 G0. In order to accurately determine the vibrational energy, 99 differential conductance spectra were collected for the junctions having a zero bias conductance of 0.1-1.2 G0. Figure 4 shows the distribution of the vibrational energy for the Co atomic wire in a hydrogen atmosphere. The histogram shows three features at around 15, 30, and 48 meV. Since the frequency of the vibrational mode at around 80 meV was quite low, this vibrational mode is not discussed further. For the clean Co contacts, the vibrational mode was observed at around 20 and 35 meV in point contact spectroscopy.16 Therefore, the vibrational modes at around 15 and 30 meV observed in the present study could be assigned to the vibrational modes of Co metals, and the vibrational mode at around 48 meV could be assigned to the vibrational mode between hydrogen and Co atomic wire. The interaction between hydrogen and Co atomic wire was revealed by IETS. Finally, the atomic structure of the Co wire in a hydrogen atmosphere is discussed based on the present experimental results and previously reported results.13,17-26 The length histogram for the last conductance plateau (see Figure 2) indicated the formation of the monoatomic wire of about two Co atoms. The hydrogen could adsorb on the surface of the Co atomic wire or be incorporated into the wire. The position of hydrogen atom or molecule could be discussed by the shape of the dI/dV curves. If the number of the conduction channel is one, the theoretical calculation predicts the conductance enhancement for contact with the conductance below 0.5 G0 and suppression above 0.5 G0.17,18 In the present study, the conductance enhancement was observed for contact with the conductance of around 1 G0 (see Figure 3b), suggesting that


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EXPERIMENTAL METHODS The measurements have been performed using the MCBJ technique. A small notch was cut in the middle of Co wires in order to fix the breaking point. The wires used were 0.1 mm in diameter, about 1 cm long. The wire was glued on top of a bending beam and mounted in a three-point bending configuration inside of a vacuum chamber. Once under vacuum and cooled to 4.2 K, the wire was broken by mechanical bending of the substrate. The bending could be relaxed to form atomic-sized contacts between the wire ends using a piezo element for fine adjustment. Hydrogen gas was admitted via a homemade capillary. About 1000 digitized conductance traces were used to build each conductance histogram in the present study. The IETS spectra were measured using a standard lock-in technique. The conductance was recorded for the fixed contact configuration using an AC modulation of 1 mV amplitude and a frequency of 7.777 kHz while slowly ramping the DC bias between -100 and þ100 mV.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: kiguti@ chem.titech.ac.jp.

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ACKNOWLEDGMENT This work was partially supported by a

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Grant-in-Aid for Scientific Research on Priority Areas “Electron transport through a linked molecule in nano-scale” from MEXT.

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Formation of Co Atomic Wire in Hydrogen Atmosphere - American

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