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Temperature dependence of Peierls–Hubbard phase transition in [Pd(cptn)2Br]Br2 studied byscanning tunneling microscopy

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2016 Jpn. J. Appl. Phys. 55 08NB16(http://iopscience.iop.org/1347-4065/55/8S1/08NB16)
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REGULAR PAPER

Japanese Journal of Applied Physics 55, 08NB16 (2016)
http://doi.org/10.7567/JJAP.55.08NB16Temperature dependence of Peierls–Hubbard phase transition
in [Pd(cptn)2Br]Br2 studied by scanning tunneling microscopy
Yuka Hosomi1, Shoji Yoshida1, Atsushi Taninaka1, Takefumi Yoshida2, Shinya Takaishi2,Osamu Takeuchi1, Masahiro Yamashita2, and Hidemi Shigekawa1*
1Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Department of Chemistry, Tohoku University, Sendai 980-8579, Japan

2*E-mail: hidemi@ims.tsukuba.ac.jp
Received January 16, 2016; revised April 3, 2016; accepted April 4, 2016; published online July 13, 2016
The temperature dependence of the Peierls–Hubbard phase transition in [Pd(cptn)2Br]Br2 (cptn: 1R,2R-diaminocyclopentane) was directly
observed using a low-temperature scanning tunneling microscope. A short ligand without alkyl chains was used to form a rigid crystal lattice toreduce the effect of structural changes in the crystal with temperature. The hysteresis in the temperature dependence of the ratio between theareas of the charge density wave (CDW) state produced by the PdII–PdIV mixed-valence state and the Mott–Hubbard (MH) state with a PdIIIaveraged valence state which is a characteristic of the ﬁrst-order phase transition, was directly observed at the atomic scale. Pinning of the CDW
phase by defects was observed below the critical temperature, suggesting the growth of the CDW phase with defects as nuclei.
© 2016 The Japan Society of Applied Physics

1.

Introduction(a)Low-dimensional materials have been extensively studied
because of their novel physical properties, for example, instrongly correlated electron systems and π-conjugate systems.
Among these materials, quasi-one-dimensional (quasi-1D)
halogen-bridged complexes (MX chains), in which isolated
1D electron systems are composed of dz2 -orbitals of metal
ions (M) and pz-orbitals of bridging halide ions (X) in M–X–
M–X– linear chain structures, have been attracting considerable attention owing to their strong Coulomb interaction
observed in phenomena such as resonance Raman spectra1–4)
and spin-Peierls transitions.5) Although more than 300
species of quasi-1D organic complexes have been synthesized, mainly using Ni or Pt metal ions, that exhibit mixed
valence states, it has been theoretically predicted that acompound complex with Pd metal will undergo a chargetransfer phase transition owing to the strong competition
between the energies of the electron–phonon interaction (S)and on-site Coulomb repulsion (U), i.e., the transfer integral(T).6–10) This is a characteristic of Pd compounds called thePeierls–Hubbard phase transition and has recently been
experimentally conﬁrmed using [Pd(en)2Br](C5-Y)2 1 H2O(en: ethylenediamine; C5-Y: dipentylsulfosuccinate), which
exhibits the phase transition between the charge density wave(CDW) and Mott–Hubbard (MH) states at 205 K.11)
Pd-MX chains form an MII–MIV mixed-valence state atroom temperature (RT), where the bridging halide ions are
displaced from the midpoints between two neighboring metal
ions as shown in Fig. 1(a). MII and MIV denote divalent andquadrivalent metals, respectively. The structure shown inFig. 1(a) is the charge density wave produced by the strained
1D structure (Peierls deformation) caused by a large S, whichforms a twofold periodicity. On the other hand, at a lowtemperature, an MIII-averaged valence state, the so-called
MH state, is formed owing to the large U, in which thebridging halogens are located at the midpoints between two
neighboring metal ions without modulation, as shown inFig. 1(b). The dz2 bands of PdII and PdIV in the CDW state
form a conduction band and a valence band, respectively,12)while the dz2 band of PdIII in the MH state splits into the(b)Fig. 1. Schematic structures of (a) CDW and (b) MH states. Arrows
indicate spins. The dz2 bands of PdII and PdIV in the CDW state form aconduction band and a valence band, respectively, while the dz2 band of PdIIIin the MH state splits into the unoccupied UH and the occupied LH, owing tothe strong interaction between electrons. EF indicates Fermi energy.

unoccupied upper Hubbard band (UH) and occupied lower
Hubbard band (LH), owing to the strong interaction betweenelectrons.
The phase transition associated with the structural change
shown in Fig. 1 has been experimentally observed by various
methods such as infrared (IR) spectroscopy, X-ray diﬀraction
analysis, and scanning tunneling microscopy (STM).11) Although these studies revealed details of the phase transition,
they were carried out for Pd complexes including ligands withlong alkyl groups. For a sample with a long alkyl chain, alarger change in the Pd–Pd distance in the b-axis direction
with temperature is considered to aﬀect the study of thecharacteristics at around the critical temperature.6–10,13,14)
Here, we present the ﬁrst results of the STM observationof the temperature dependence of the structural change ofCDW-MH phases in [Pd(cptn)2Br]Br2 (cptn: 1R,2R-diaminocyclopentane). A sample with a short ligand without alkyl
chains was used in our experiment to form a rigid crystal
lattice to attenuate the eﬀect of structural changes in thecrystal with temperature.

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Jpn. J. Appl. Phys. 55, 08NB16 (2016)

Y. Hosomi et al.

Fig. 3. (Color) STM images of [Pd(cptn)2Br]Br2 surfaces obtained at(a) RT (Vs = 1.5 V, It = 0.08 nA) and (b) 103 K (Vs = 4.0 V, It = 0.1 nA).
Fig. 2. (Color online) Schematic of the crystal structure of[Pd(cptn)2Br]Br2 (cptn: 1R,2R-diaminocyclopentane).

2.

Experimental procedure

Figure 2 shows the crystal structure of [Pd(cptn)2Br]Br2, anoctahedral structure consisting of four N atoms (two ligands)
and two Br atoms placed around a Pd atom, in which the dz2
and dx2 y2 orbitals respectively extend vertically along theb-axis and in the four side directions. The dz2 orbital of Pd
and the pz orbital of Br strongly overlap and are alternatelyordered to form a quasi-one-dimensional (quasi-1D) halogenbridged complex.
The [Pd(cptn)2Br]Br2 crystal has a unit cell of a = 2.39 nm,
b = 0.707 nm, and c = 0.529 nm, which develops a twofold periodicity along the b-axis in the CDW phase, andcan be directly observed by STM.14–22) Single-crystal[Pd(cptn)2Br]Br2 samples were grown by the electrochemical
oxidation method,23) which were cut and ﬁxed on a sample
stage with carbon paste. Experiments were carried out using
an Omicron low-temperature scanning tunneling microscopewith a base pressure of 10−9 Pa. The performance of a piezo
scanner at low temperatures was calibrated using a Si(001)surface structure. Measurement below 100 K was notpossible in our experiment because of the diﬃculty inmaking the microscope tip approach the sample owing tothe large decrease in the conductivity in the MH state. Inaddition, when a negative bias voltage was applied to thesample, the sample surface was damaged, which may havebeen caused by the reduced number of carriers in the surface
region of the sample. Although the details are not yet clear,
all measurements reported in this paper were carried out witha positive bias voltage applied to the sample.
The sample temperature, which was measured by athermocouple attached to the sample stage, was controlledusing a resistive heater attached near the sample stage, which
was cooled using liquid N2. Thermal drift was suppressed bykeeping the sample at each temperature for a suﬃcient time
before measurements, which varied from 1–2 h to overnight
depending on the conditions.
3.

Results and discussion

Figure 3 shows typical STM images of the [Pd(cptn)2Br]Br2b–c plane obtained at (a) RT and (b) 103 K, where the Pd
atoms shown in Fig. 2 are imaged as bright spots similarly tothat in a previous paper.11,14) Cross sections along the bluelines in the STM images are shown together, where L and R
show the positional relationship. As shown in Fig. 3, theperiodicities along the b- and c-axes are (a) (0.98 nm,0.68 nm) and (b) (0.49 nm, 0.69 nm), respectively. Since theSTM image showing the twofold periodicity is similar to thatobtained at RT in a previous study and the original
periodicity appeared at lower temperatures, as expected, weattributed those structures to the PdII–Br–PdIV CDW state at
RT and to the PdIII–Br–PdIII MH state at 103 K, respectively.
The bright protrusions were attributed to the Pd dz2 orbital,
similarly to those in the previous image obtained at RT.
Next, we directly observed the temperature dependenceof the phase transition. Figure 4 shows the change in theSTM images of the [Pd(cptn)2Br]Br2 b–c plane withdecreasing [(a) to (c)] and increasing [(d) and (e)] temperature. The area ratios between the CDW and MH phases
shown above the STM images were obtained from theaverage of the STM images measured over 25 × 25 nm2 areasat diﬀerent locations at each temperature. First, the temperature was stabilized at 155 K by adjusting the current ofthe heater, and the required images were taken. Then thetemperature was decreased to 113 and 103 K and thenincreased to 115 and 158 K in a stepwise manner by changing
the current of the heater, and images were taken at eachtemperature. Thermal drift was suppressed by keeping thesample at each temperature for a suﬃcient time before
measurements, as described above, which also ensured
accurate temperature control. Cross sections along the lines
in the STM images are shown together.
As shown in Figs. 4(a) and 4(e), almost the entire surface
was occupied by the 2 × 1 CDW phase at 155 K, while, as
shown in Fig. 4(c), almost the entire surface was covered by
the MH state at 103 K; the CDW=MH area ratio was 1 : 9,
conﬁrming that the CDW and MH phases appear at highand low temperatures, respectively. Here, the CDW phase
comprising ∼1=10 of the surface area at 103 and 115 K wasobserved around the defects, as shown in Figs. 4(c) and 4(d),respectively. Figure 4(f) shows a cross section along the 1D
chain of Pd atoms surrounded by the dashed rectangle in the08NB16-2© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 08NB16 (2016)

Y. Hosomi et al.

Fig. 4. (Color) Variation in the STM images of the [Pd(cptn)2Br]Br2 surface with decreasing (a–c) and increasing (d, e) sample temperature, showing thetemperature dependence of the phase transition between the CDW and MH phases. The measurement conditions ðVs ; It Þ were (a) (2.0 V, 0.2 nA), (b) (3.5 V,0.1 nA), (c) (4.0 V, 0.08 nA), (d) (3.3 V, 0.08 nA), and (e) (3.8 V, 0.08 nA). The area ratios between the CDW and MH phases shown above the STM images
were obtained from the average of the STM images measured over 25 × 25 nm2 areas at diﬀerent locations at each temperature. (f) Cross section along the 1D
chain surrounded by the dashed rectangle in (d).

left of Fig. 4(d), where only one defect exists in the chain,showing a gradual change in the structure from the CDW
phase near the defect located near letter L (bottom left) to
the MH phase along the chain (L–R). These results suggestthat the CDW phase tends to be pinned by defects at lowtemperatures and grows with increasing temperature. As
shown in Figs. 4(c) and 4(d), the eﬀect of defects on theneighboring chains looks weak, which, however, may aﬀect
the critical temperature. This eﬀect may be due to the strainaround the defects, although it has not yet been conﬁrmed.
Another noteworthy point is that the CDW=MH area ratio
at around 115 K was diﬀerent between the surfaces observed
upon the temperature being decreased [(b) 113 K, 6 : 4] andincreased [(e) 115 K, 1 : 9], i.e., although the latter temperature is slightly higher, the ratio of the CDW phase is lower.
The MH state remains even at a higher temperature when
the temperature is increased. The hysteresis, which is acharacteristic of the ﬁrst-order phase transition and wasobserved in a previous work by macroscopic methods such as
Raman spectroscopy,11) was directly observed at the atomic
scale for the ﬁrst time.

As reported in a previous paper, the Pd–Pd distance in theb-axis direction becomes closer for a sample with a longer
alkyl chain, which increases critical temperature by ∼30 K
when one C atom in the alkyl chain is added.13) As expected,
the critical temperature of ∼100 K observed in this experiment for the sample without alkyl chains is lower than thatobserved for [Pd(en)2Br](C5-Y)2 1 H2O of 205 K,13) which isof a similar order despite their structures being diﬀerent fromeach other. Theoretical calculations are necessary to obtainbetter understanding of these results. The analysis of thegrowth of the CDW phase in the intermediate state isimportant to clarify details of the phase transition, such as
the interactions in 1D chains and between 1D chains. The
analysis of the ﬂuctuations of the two phases and the eﬀect ofalkyl chains will also be an interesting topic for future study.
These analyses are under consideration.
4.

Conclusions

We succeeded in the direct observation by STM of thephase transition between the CDW and MH phases in[Pd(cptn)2Br]Br2, in which a short ligand without alkyl

08NB16-3© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 08NB16 (2016)

Y. Hosomi et al.

chains was used to form a rigid crystal lattice to reduce theeﬀect of structural changes with temperature. The hysteresis
in the temperature dependence of the ratio between the CDW
and MH phase areas, which is characteristic of the ﬁrst-orderphase transition, was directly observed at the atomic scale.
The pinning of the CDW phase by defects was observed
below the critical temperature, suggesting the growth of theCDW phase with defects as nuclei.

8)
9)
10)
11)
12)

13)

14)

Acknowledgment

H.S. acknowledges the support from the Japan Society for thePromotion of Science (Grant-in-Aid for Scientiﬁc Research,
15H05734).
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© 2016 The Japan Society of Applied Physics