Spectroscopic study of low-temperature hydrogen absorption in palladium
K. Ienaga, H. Takata, Y. Onishi, Y. Inagaki, H. Tsujii, T. Kimura, and T. Kawae
Citation: Applied Physics Letters 106, 021605 (2015); doi: 10.1063/1.4905729
View online: http://dx.doi.org/10.1063/1.4905729
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/2?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Novel insight into the hydrogen absorption mechanism at the Pd(110) surface
J. Chem. Phys. 140, 134705 (2014); 10.1063/1.4869544
Inelastic electron tunneling spectroscopy study of ultrathin Al 2 O 3 – TiO 2 dielectric stack on Si
Appl. Phys. Lett. 97, 202905 (2010); 10.1063/1.3518478
Imaging and vibrational spectroscopy of single pyridine molecules on Ag(110) using a low-temperature scanningtunneling microscope
J. Chem. Phys. 124, 204708 (2006); 10.1063/1.2200350
Superconductivity of nanometer-size Pb islands studied by low-temperature scanning tunneling microscopy
Appl. Phys. Lett. 88, 113115 (2006); 10.1063/1.2184758
Low-temperature dynamics and spectroscopy in exohedral rare-gas C 60 fullerene complexes
J. Chem. Phys. 114, 5156 (2001); 10.1063/1.1350918

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Tue, 23 Aug 2016
10:50:22

APPLIED PHYSICS LETTERS 106, 021605 (2015)

Spectroscopic study of low-temperature hydrogen absorption in palladium
K. Ienaga,1,a) H. Takata,1 Y. Onishi,1 Y. Inagaki,1 H. Tsujii,2 T. Kimura,3 and T. Kawae1
1

Department of Applied Quantum Physics, Faculty of Engineering, Kyushu University, Motooka, Nishi-Ku,
Fukuoka 819-0395, Japan2
Department of Physics, Faculty of Education, Kanazawa University, Kakuma-machi, Kanazawa 920-1192,
Japan
3
Department of Physics, Kyushu University, Hakozaki, Higashi-Ku, Fukuoka 812-8581, Japan(Received 12 November 2014; accepted 26 December 2014; published online 14 January 2015)
We report real-time detection of hydrogen (H) absorption in metallic palladium (Pd) nano-contacts
immersed in liquid H2 using inelastic electron spectroscopy (IES). After introduction of liquid H2,
the spectra exhibit the time evolution from the pure Pd to the Pd hydride, indicating that H atoms
are absorbed in Pd nano-contacts even at the temperature where the thermal process is notexpected. The IES time and bias voltage dependences show that H absorption develops by applying
bias voltage 30  50 mV, which can be explained by quantum tunneling. The results represent that
C 2015IES is a powerful method to study the kinetics of high density H on solid surface. V
AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905729]

Hydrogen (H) is ideal replacement for synthetic fuelbecause of its lightest weight, high abundance, high energy
density, and environmental cleanness of the oxidation product. Due to its high chemical reactivity, absorption into metals is a safe way. Moreover, several types of metalcompounds can contain H with higher density than liquid H2(LH2).1 For the industrial applications, elemental processesof H absorption including chemisorption, penetration, anddiffusion have been investigated in many metallic systems.2–4 However, the investigation of H absorption into
metal is a challenging issue because of the strong quantum
character of H. Actually, quantum phenomena of H, such as
quantum tunneling (QT), is reported in metal-hydride and on
solid surface.4–9
The kinetics of H on solid surfaces have been investigated with a variety of experiments in ultra-high vacuum
environment (e.g., electron diffraction, work function
measurements, thermal desorption mass spectrometry,and scanning probe microscopy),3,6,10–12 and theoretical
calculations.3,9,13–15 These results show that H occupationon the surface or subsurface sites is more favorable thanthat in inner bulk, suggesting that H absorption into bulk is
restricted by surface activation barrier at low temperature.
On the other hand, some recent experiments suggest that
exposure of the surface to high-pressure H2 urges H penetration into the inside.16–18 These facts indicate that exploring the interaction between LH2 or high-density H2 gas andmaterials is inevitable to advance H storage technology.
In this Letter, we report the real time detection of H
absorption into palladium (Pd) immersed in LH2 throughinelastic electron spectroscopy (IES) measurements for Pd
nano-contact. When the contact regime is smaller than themean free path of conduction electrons, the electrons aretransferred through the steep potential drop without energy
dissipation in the regime (ballistic electron), and then losea)

Present address: Institute for Solid State Physics, The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8581, Japan; Electronic mail:ienaga@issp.u-tokyo.ac.jp0003-6951/2015/106(2)/021605/4/$30.00

their energy by scattering with excitations such as phonon as
illustrated in Fig. 1(a).19 This leads to a peak anomaly,
corresponding to the state density of the excitation, in the
differential conductance (dI/dV) spectra. The “ballistic” electron spectroscopy has been employed for the detection ofelectron-phonon interaction in metallic nano-contacts,20–22
and vibrational modes of a single molecular bridge.23 In thepresent study, it is used for the detection of vibrationalmodes of absorbed H atoms. From the IES time and bias
voltage dependences of Pd nano-contacts, we reveal that H
absorption develops by applying bias voltage 30–50 mV,
which can be understood by QT.
We prepare Pd nano-contacts with a mechanically controllable break junction (MCBJ) technique,19 thus avoiding
surface contamination and oxidation. Pd polycrystalline wire
(99.998%, 0.2 mm diameter) is stretched by bending aphosphor-bronze beam with a screw and a piezo element invacuum at low temperatures. Note that the Pd nano-contact
diameter varies from nanometers to micrometers and is kept
constant over several hours by preventing thermal input.24,25
To introduce LH2, the MCBJ apparatus is installed in an
inner cell having flexible bellows inside the vacuum chamberas shown in Fig. 1(b).26
In Figs. 2(a)–2(f), we show dI/dV spectra and the derivative d2I/dV2 spectra, for different Pd nano-contacts, measured as a function of bias voltage using a lock-in technique(1 kHz modulation). The data indicate the efficacy of IES for

FIG. 1. (a) Energy diagram for ballistic transport of conduction electrons in
a Pd nano-contact. (b) Schematic of the MCBJ cryostat. H absorption anddiffusion experiments can be carried out in LH2, avoiding the temperature
increase due to Joule heating in the nano-contacts during the measurements.

106, 021605-1

C 2015 AIP Publishing LLC
V

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Tue, 23 Aug 2016
10:50:22

021605-2

Ienaga et al.

FIG. 2. (a) dI/dV spectrum for a pure Pd nano-contact in vacuum at 18 K,
where the vertical axis is the universal conductance unit G0 ¼ 2e2 =h. (b)d2I/dV2 spectrum, which is the numerical derivative of (a). (c) dI/dV spectrum of Pd hydride nano-contacts in vacuum at 18 K. (d) d2I/dV2 from thedI/dV spectrum of Pd hydride nano-contacts. (e) The time evolution of dI/dV
spectra of Pd immersed in LH2. For comparison, the spectrum in (a) is plotted in (e) as a dashed line. (f) d2I/dV2 spectra of (e). The spectra 1–4 in (e)and (f) are obtained at 500 s, 2500 s, 4500 s, and 8000 s after immersion in
LH2, respectively.

the detection of H absorption. In the spectra of pure Pd(Figs. 2(a) and 2(b)), the peak in d2I/dV2 at jVj  20 mV iscaused by electron-phonon scattering.20–22 The spectra of Pd
hydride in vacuum are shown in Figs. 2(c) and 2(d). To prepare Pd hydride, Pd wire is exposed to 1 atm of H2 gas in the
inner cell at 300 K for a few hours, creating a hydrogen concentration of 0.6.27 Then, the H2 gas is pumped out of the
MCBJ apparatus at 80 K to prevent H desorption from the
Pd. The nano-contact is formed by stretching the Pd hydrideat 18 K. The spectra in Figs. 2(c) and 2(d) differ from thoseof pure Pd in that a new peak at jVj  60 mV is present andcomes from electron scattering with vibrational modes of H
atoms in fcc Pd octahedral sites.8,23,28
After preparing the pure Pd nano-contact in vacuum at
18 K, the inner cell is filled by LH2. Then, sweep measurements of jVj  80 mV are performed repeatedly at 18 K. The
dI/dV and d2I/dV2 spectra exhibit the time evolution shownin Figs. 2(e) and 2(f). The spectra clearly evolve from pure
Pd in Figs. 2(a) and 2(b) to the hydride spectra in Figs. 2(c)and 2(d); the peak at jVj  20 mV is suppressed while thenew peak at jVj  60 mV grows. These demonstrate that H
is gradually absorbed by the Pd up to its 18 K limit. Notethat the background of electron-phonon interaction superposes on the spectra above jVj  20 mV, which covers thegrowth of the peak at jVj  60 mV in Fig. 2(f). The spectrum
3 in Fig. 2(f) is nearly identical to that in Fig. 2(d), indicatingthat the final H concentration is expected to be higher than
0.6. Given that superconductivity in the PdHx systems isAppl. Phys. Lett. 106, 021605 (2015)

not observed at temperatures above 1.5 K, H concentrationmust be x < 0.84.29
We closely examined the time evolution of the peak amplitude at 20 mV for the Pd nano-contact immersed in LH2,
because the peak reflecting the electron-phonon interaction issensitive to the amount of H, as shown in Fig. 2(f) and as discussed later. Moreover, the bias effect is not large at this voltage. In these measurements, the contact diameter is kept 10 nmwith feedback adjustment, corresponding to G(0)  1000 G0,
where G(0) is the zero-bias conductance obtained from dI/dV
at 0 mV. We sweep the bias for jVj  20 mV only three times:
initially, at 2000 s, then at 40 000 s, after introduction of LH2,
while keeping jVj ¼ 0 mV otherwise. As seen in the spectra ofFigs. 3(a) and 3(b), the d2I/dV2 amplitude decreases only at2000 s, with no decrease at 40 000 s.
The amplitude at 20 mV in Fig. 3(b) is much larger than
the full absorption at T ¼ 18 K in Fig. 2(f), suggesting that thePd sample has room to absorb more H atoms and that the bias
voltage is related to the H absorption. Thus, we examine thebias effect for H absorption by applying a constant bias Vconst¼ 20 mV during the measurement for the same sample used in
Figs. 3(a) and 3(b). Then, we increase Vconst to 60 mV in
10 mV steps. The time evolution of the d2I/dV2 amplitudeat each bias voltage is plotted in Fig. 3(e). At Vconst ¼ 20 mV,
d2I/dV2 is almost constant, indicating that further absorption
does not occur. However, for Vconst ¼ 30 mV, 40 mV, and
50 mV, d2I/dV2 decreases with time. At Vconst ¼ 60 mV, d2I/
dV2 is a constant value that is almost identical to that of

FIG. 3. (a) dI/dV spectra before, at 2000 s, and at 40 000 s after the introduction of LH2. (b) d2I/dV2 spectra of (a). (c) dI/dV spectra marked by arrows in(e). (d) d2I/dV2 spectra of (c). (e) The time and bias dependence of the
20 mV peak height in the spectra, measured by sweeping the bias in jVj
 Vconst every 1000 s, and taken over 16 h at each Vconst, where the conductance is 1000 G0. The inset shows the sequence of the bias sweep.

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Tue, 23 Aug 2016
10:50:22

021605-3

Ienaga et al.

curve 4 in Fig. 2(f). The amplitudes indicated by arrows in Fig.
3(e) correspond to the spectra in Figs. 3(c) and 3(d). Note thatthe spectra in Fig. 3(d) evolve similarly as those in Fig. 2(f)do. From these results, it is reasonable to conclude thatH absorption occurs because of the applied bias voltage.
Moreover, the amount increases monotonically with the bias
voltage and saturates at jVj  60 mV.
To determine the location of the absorbed H atoms (i.e.,on the Pd surface or in the bulk), we investigated thecontact-size dependence of d2I/dV2 spectra in LH2, as wellas that for pure Pd. The results are plotted in Fig. 4(a) as afunction of G(0). The amplitudes at 20 mV in LH2 are muchsmaller than those of pure Pd and close to those of Pd
hydride at each contact size. In Figs. 4(b) and 4(c), we plot
the amplitude at 20 mV and 60 mV as a function of thecontact diameter d, estimated from G(0) using the Sharvinrelation.30 The amplitudes at 20 mV of pure Pd are proportional to d3.2, which is understood in terms of the volume dependence of the acoustic phonon vibration in a pure Pd
crystal.31 In contrast, the amplitudes at 20 mV of Pd hydridechange to d2.4 dependence, which indicate that the peak issuppressed with the increase in d. The decrease of the power
is probably explained by the suppression of electron-phonon
interaction due to H absorption, which is observed in electronic resistivity measurements.32 Since volume/surface ratio
increases with increasing d, the ratio of H occupation sites to
Pd atoms constructing the contact increases with d. As aresult, the power is decreased from 3.2 to 2.4 by H absorption in Pd contact.

FIG. 4. (a) The amplitude in d2I/dV2 at 20 mV for pure Pd nano-contacts(open circle), for Pd hydrides (square), for Pd nano-contacts in LH2 measured just after the introduction of H2 (cross), and after the application of the
60 mV bias voltage in Fig. 3(d) (full circle). They are plotted as a functionof zero-bias conductance G(0). (b) Plot of the same data in (a) as a functionof the diameter d, which is estimated from the conductance. (c) The size
dependences of the amplitude in d2I/dV2 at 60 mV for Pd hydrides, and Pd
nano-contacts in LH2 after the application of the 60 mV bias voltage in
Fig. 3(d). The solid lines are least-squares fits.

Appl. Phys. Lett. 106, 021605 (2015)

In the case of Pd in LH2, it is important to note that theamplitudes at 20 mV are proportional to d2.9 just after introducing the LH2. After saturation of H absorption, the amplitudes at 20 mV show d2.2 dependence, which is close tothat of Pd hydride. Moreover, the d-dependences of amplitudes at 60 mV for Pd nano-contacts in LH2 and Pd hydrides
are almost the same as shown in Fig. 4(c). The change ofpower for Pd nano-contacts by introduction of LH2 is well
explained by the development of H absorption from nearsurface region to inner bulk. From these results, we conclude
that, after applying the bias voltage, H atoms are absorbed
uniformly in the bulk of the Pd contact.
Generally, H absorption in metals occurs in three steps:
H chemisorption from dissociated H2 molecules on the surface, H penetration from the surface, and H diffusion in thebulk. Even at low temperatures, the dissociation of H2 into H
atoms proceeds on the Pd surface spontaneously because of
the energy difference between the two states.3 Therefore, H
atoms are sufficiently supplied at the Pd surface in LH2.
Moreover, the spectra in Figs. 3(a) and 3(b) change soon after introducing LH2, and the absorption develops dependingon the bias voltage. These results clearly reflect the later two
steps, H penetration and H diffusion in the bulk. The rapidchange in Figs. 3(a) and 3(b) corresponds to the H penetration from the surface into near-surface region due to highpressure effects.16,17 Taking into account that the H diffusion
in bulk is rate-limiting process when H2 pressure is sufficiently high,11 the time and bias dependences of IES spectrain Figs. 3(c)–3(e) are reasonably understood by H diffusion
in bulk Pd.
Thermal hopping and QT are the two possible diffusion
modes. In thermal diffusion, H atoms in fcc octahedral Pd
sites must acquire the energy to surmount the activation barrier Ediff  226 meV.8 The diffusion coefficient is given by
D ¼ D0exp(Ediff /kBT), where D0 ¼ 2.9  107 m2s for Pd
and kB is Boltzmann’s constant. At T ¼ 18 K, D  1070 m2s,
suggesting that the thermal process is highly unlikely. QT
can explain the bias dependence shown in Fig. 3(e) as follows. Because of lattice strain, H atoms undergo the “selftrap effect” in Pd, as illustrated in Fig. 5(a), where they are
initially 50 meV lower in energy than corresponding levelsin the absence of H atoms.7 QT diffusion occurs when an H
atom tunnels through the barrier of the potential well trapping the H atom into the well corresponding to absence of H,
as shown in Fig. 5(b). This process is assisted by phonons,
which raise the energy of the trapped H atoms.7,8 Because
nano-contacts prepared by MCBJ inevitably have lattice

FIG. 5. (a) The self-trapped state of an H atom, where the energy level ofthe atom is lowered by 50 meV as a result of lattice strain. (b) The diffusion process from the self-trapped state, where the H atom is excited by phonons created by scattered inelastic electrons, and can tunnel to theneighbouring site.

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Tue, 23 Aug 2016
10:50:22

021605-4

Ienaga et al.

disorders, it is most likely that the bias dependence for theamount of H absorption in Fig. 3(e) is derived from the distribution of potential depths. Moreover, it seems that the
spatial distribution changes the relaxation speed, as shown21 2
in Fig.p3(e).
ﬃﬃﬃﬃﬃﬃﬃﬃﬃWe estimate that D  10 m s by the equation,
RD ¼ DDt, where the contact radius RD and the relaxation
time Dt are 5 nm at 1000 G0 and 10 000 s at Vconst¼ 40 mV, respectively. This D is considerably larger thanthat for thermal diffusion at 18 K, as estimated above, indicating that the diffusion is governed by QT.
In summary, we study H atom absorption and diffusion in Pd nano-contacts immersed in liquid H2 using IES
measurements. The IES time and bias voltage dependences demonstrate that H absorption develops by applying
the bias voltage 30–50 mV, which can be explained by
QT process assisted by phonons induced by ballistic electrons. The present results indicate that QT of atoms canbe studied by transport measurements as done in electronic systems, where many characteristic features havebeen revealed. Moreover, IES is a powerful method toinvestigate H diffusion in metals exposed to LH2 or highpressure H2.
This work was supported by a Grant-in-Aid for
Scientific Research (Grant Nos. 25220605, 25287076, and26600102) from the Japan Society for the Promotion of
Science. K.I. is financially supported by the Japan Society
for the Promotion of Science for Young Scientists (Grant
No. 12J06024).
1

L. Schlapbach and A. Z€
uttel, Nature 414, 353 (2001).
A. Pundt and R. Kirchheim, Annu. Rev. Mater. Res. 36, 555 (2006).
3
K. Christmann, Surf. Sci. Rep. 9, 1 (1988).
4
Y. Fukai, The Metal-Hydrogen System, 2nd ed. (Springer, Berlin, 2005).
5
R. R. Arons, Y. Tamminga, and G. de Vries, Phys. Status Solidi 40, 107(1970).
6
L. J. Lauhon and W. Ho, Phys. Rev. Lett. 85, 4566 (2000).
2Appl. Phys. Lett. 106, 021605 (2015)
7

C. P. Flynn and A. M. Stoneham, Phys. Rev. B 1, 3966 (1970).
Y. Fukai and H. Sugimoto, Adv. Phys. 34, 263 (1985).
N. Ozawa, N. B. Arboleda, Jr., T. Roman, H. Nakanishi, W. A. Din~ao, and
H. Kasai, J. Phys. Condens. Matter 19, 365214 (2007).
10
G. E. Gdowski, T. E. Felter, and R. H. Stulen, Surf. Sci. 181, L147
(1987).
11
H. Okuyama, W. Siga, N. Takagi, M. Nishijima, and T. Aruga, Surf. Sci.
401, 344 (1998).
12
E. C. H. Sykes, L. C. Fernandez-Torres, S. U. Nanayakkara, B. A.
Mantooth, R. M. Nevin, and P. S. Weiss, Proc. Natl. Acad. Sci. U.S.A.
102, 17907 (2005).
13
P. Ferrin, S. Kandoi, A. U. Nilekara, and M. Mavrikakis, Surf. Sci. 606,
679 (2012).
14
B. Hammer and J. K. Nørskov, Surf. Sci. 343, 211 (1995).
15
O. M. Løvvik and R. A. Olsen, Phys. Rev. B 58, 10890 (1998).
16
S. Ohno, M. Wilde, and K. Fukutani, J. Chem. Phys. 140, 134705(2014).
17
A. Groß, Chemphyschem 11, 1374 (2010).
18
M. Wilde and K. Fukutani, Phys. Rev. B 78, 115411 (2008).
19
N. Agra€ıt, A. L. Yeyati, and J. M. van Ruitenbeek, Phys. Rep. 377, 81(2003).
20
I. K. Yanson and Yu. N. Shalov, Sov. Phys. JETP 44, 148–155 (1976).
21
A. G. M. Jansen, F. M. Mueller, and P. Wyder, Science 199, 1037–1040(1978).
22
Y. G. Naidyuk and I. K. Yanson, Point-Contact Spectroscopy (Springer,
Berlin, 2004).
23
S. Csonka, A. Halbritter, G. Mihaly, O. I. Shklyarevskii, S. Speller, and H.
van Kempen, Phys. Rev. Lett. 93, 016802 (2004).
24
K. Ienaga, N. Nakashima, Y. Inagaki, H. Tsujii, S. Honda, T. Kimura, and
T. Kawae, Phys. Rev. B 86, 064404 (2012).
25
K. Ienaga, N. Nakashima, Y. Inagaki, H. Tsujii, T. Kimura, and T. Kawae,
Appl. Phys Lett. 101, 123114 (2012).
26
K. Ienaga, T. Yokota, N. Nakashima, Y. Inagaki, H. Tsujii, and T. Kawae,
J. Phys.: Conf. Ser. 400, 042019 (2012).
27
E. Wicke and H. Brodowsky, Hydrogen in Metals II, edited by G. Alefeld
and J. Voelkl (Springer, Berlin, 1978).
28
J. M. Rowe, J. J. Rush, H. G. Smith, M. Mostoller, and H. E. Flotow,
Phys. Rev. Lett. 33, 1297 (1974).
29
R. W. Standley, M. Steinback, and C. B. Satterthwaite, Solid State
Commun. 31, 801 (1979).
30
Y. V. Sharvin, Sov. Phys. JETP 21, 655 (1965).
31
I. K. Yanson, V. V. Fisun, R. Hesper, A. V. Khotkevich, J. M. Krans, J. A.
Mydosh, and J. M. van Ruitenbeek, Phys. Rev. Lett. 74, 302 (1995).
32
H. Akaki, Y. Sakamaki, S. Harada, and M. Kubota, J. Alloys Compd. 446,
436 (2007).
8
9

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Tue, 23 Aug 2016
10:50:22