Metal–semiconductor transition in atomically thin Bi2Sr2Co2O8
nanosheets
Yang Wang1, Rui Cheng1, Jianjin Dong1, Yuan Liu1, Hailong Zhou2, Woo
Jong Yu2, Ichiro Terasaki3, Yu Huang1,4, †, Xiangfeng Duan2,4, a
1Department
of Materials Science and Engineering, University of California, Los Angeles, California 90095,
USA
2Department
of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA
3Department
of Physics, Nagoya University, Nagoya 464-8602, Japan
4California
a)
Nanosystems Institute, University of California, Los Angeles, California 90095, USA
Electronic mail: xduan@chem.ucla.edu
Supporting Information
1. Low-temperature localization in BSCO bulk and N≥4 nanosheets. (FIG. S1)
2. Transport properties of BSCO nanosheets. (FIGS. S2-S5)
3. Raman spectroscopy results. (FIG. S6)
4. The effect of SiO2/Si substrate on BSCO.
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1.
Low-temperature localization in N≥4 nanosheets and bulk BSCO
The N≥4 nanosheets and bulk BSCO have almost the identical transport characteristics.
They typically exhibit a metallic behavior in resistance-temperature dependence at higher
temperatures. With decreasing temperature, the localized spins induced by the interlayer
magnetic correlation result in a localized transport behavior. As a result, the metallic conduction
does not persist to the low temperatures. In contrast, for the few-layered nanosheets (N<4), with
weak interlayer coupling and the absence of localized interlayer magnetic correlation, we should
expect that the metallic conduction could persist at low temperature. However, this is not the
case. Our results show that these few-layered samples (N<4) exhibit much stronger localization
throughout the temperature of 2 to 300 K, which can be attributed to disorder potential and
Coulomb charging effect. Therefore, the localized characteristics of N<4 nanosheets and N≥4
nanosheets
(and
bulk)
have
totally
different
origins.
This
supported
by
the
temperature-dependent transport characteristics. In particular, the plot of lnG as a function of
T-1/3 shows that the N=4 sample in the semiconducting-like range (below 140 K) does not obey
the Mott-VRH mechanism (Fig. S1), in stark contrast to the cases of N=1, 2, and 3 samples.
Furthermore, the G-T behavior obeys neither ES-VRH nor thermal-activation-type conduction.
This supports that the localization below 140 K in the N=4 sample is not originated from
disorder potential or Coulomb charging effect, which is different from the cases of N=1, 2, and 3
samples.
FIG. S1. (a) Temperature dependence of conduction G of the N=4 nanosheet. (b) The
corresponding logarithmic conductance lnG as a function of T-1/3.
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2.
Transport properties of BSCO nanosheets
FIG. S2. Temperature dependence of transport behavior of BSCO nanosheets. (a-d) Ids-Vds
curves of N=4, 3, 2, and 1 nanosheets under Vbg = 0 measured at different temperatures.
FIG. S3. Temperature dependence of transport behavior of BSCO monolayer. (a) Temperature
dependence of sheet resistance R (=1/G). (b) Temperature dependence of mobility μ, calculated
by  = dIds /dVbg
 [L/(WCi Vds )] with the capacitance between the channel and back-gate per
max
unit area Ci = 1.18×10-4 Fm-2.
3
FIG. S4. Temperature dependence of transport behavior of BSCO nanosheets under Vbg = 0. (a-d)
Temperature dependence of sheet conductance G of BSCO nanosheets. The G-T behavior under
Vbg = 0 is consistent with that under Vbg = -84 V. The N=4 nanosheets have similar electric
transport behavior with the BSCO bulk, whereas the N=3, 2, and 1 nanosheets remain
semiconducting behavior throughout the temperature range. (e-h) Solid lines: Ids-Vds curves of
BSCO nanosheets at 10 K (at 2 K for the quadrilayer) under Vbg = 0 in a small range of ±20 mV.
Dash line: the corresponding dIds/dVds curves (for the monolayer, the dIds/dVds is shown as black
squares and the dash lines are guide for eyes). Similar with the case under Vbg = -84 V, the
nonlinear conductance suppression is observed in monolayer, bilayer, and trilayer but it is absent
in quadrilayer. Under zero gate, the scale of the drain source gap is slightly larger than that under
-84 V gate. ∆bias is 6.1, 1.4, and 0.6 meV for the monolayer, bilayer, and trilayer, respectively, as
denoted by the arrows.
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FIG. S5. The fittings of conductance under Vbg = -84 V vs temperature. (a,b) Logarithmic
conductance lnG as a function of T-1/3 of BSCO trilayer and bilayer, respectively. The solid lines
are linear fittings. Similar with the case under zero gate, the linear relationship between lnG and
T-1/3 is valid throughout the whole temperature range, indicating the Mott 2D-VRH transport
mechanism. (c) Plot of lnG vs T-1/3 of BSCO monolayer, where the linear relationship (Mott
2D-VRH) is valid from 300 to ~55 K. (d) Plot of lnG vs T-1/2 of monolayer. The linear
relationship below ~55 K reveals an ES-VRH mechanism in this temperature range. The dash
lines in (c) and (d) are guide for eyes. It can be seen that the transport behaviors under Vbg = -84
V are all similar with the case under zero gate, but the characteristic temperatures (TM, TES, and
Tcross) become smaller, indicating that the disorder is partially suppressed under a large negative
gate voltage.
3. Raman spectroscopy spectra of BSCO.
According to the Raman spectra (Fig. S5a), the two phonon peaks at ~440 cm-1 and ~615 cm-1,
corresponding to the E1g and A1g modes, represent the in-plane and out-of-plane vibrations of
oxygen atoms, respectively (1). The A1g peak can be extracted by subtracting the background of
silicon (Fig. S5b). As the layer number decreases, the intensity of out-of-plane vibration mode
A1g only slightly decreases until N=3, where it suddenly weakens (Fig. S5c), suggesting a
discontinuous interlayer exchange coupling happens at N=4 to 3.
From the Raman peaks, the strength of electron-phonon coupling λ can be extracted by
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I  I 0 [1   (   0 ) / ]2 /[1  (   0 ) 2 /  2 ] ,
where ω0 is the discrete phonon frequency, I and Γ are intensity and width of the phonon peak
(2). By fitting the Raman spectra, the extracted λ values are presented in Fig. S5d. It can be seen
that with reducing layer number, λ increases slowly until N=4, which means that the
electron-phonon interaction in this system is less affected by layer number for the N≥4 flakes.
However, λ suddenly increases with the change of N=4 to 3 as well as N=2 to 1. Since
electron-phonon coupling is crucial for the formation of polarons and thus induces the
localization of electrons in semiconductors, herein the noticeable enhancement of λ indicates a
large enhancement of carrier localization at N=4 to 3 and N=2 to 1.
FIG. S6. Raman spectra and electron-phonon coupling strength of the BSCO system. (a)
Normalized Raman spectra of BSCO nanosheets and bulk. The measurement was performed at
room temperature under ambient conditions by a Renishaw 1000 system. The two phonon peaks
at ~440 cm-1 and ~615 cm-1 represent the in-plane and out-of-plane vibration modes E1g and A1g,
respectively. For the nanosheets, the strong Raman scattering at 520 cm-1 is from silicon
substrate. Raman signal is relatively weak in few-layered BSCO because less material is excited,
so the Raman spectra are normalized by the E1g peak for comparison. (b) Fitting of the silicon
Raman peak for the N=1 nanosheet. By subtracting the background of silicon, the A1g peak of
BSCO nanosheets can be extracted. (c) Comparison of the A1g peak of BSCO nanosheets and
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bulk. (d) Layer number N dependence of the electron-phonon coupling strength λ; the dash lines
are guide for eyes.
4.
The effect of SiO2/Si substrate on BSCO
Disorder effect caused by underlying SiO2/Si substrate has been widely reported in 2D
materials. This disorder can be screened better with increasing layer number, which could partly
contribute to the layer-dependent transport behavior. Nonetheless, it is important to note that
SiO2 disorder potential likely plays much a less important role in the BSCO system than in
graphene or MoS2 for several reasons. First, the surface roughness of SiO2/Si substrate is
typically around ±0.2 nm (3), whereas the thickness of BSCO single-layer is 3 nm. Therefore,
different from the case of graphene or MoS2, of which the single-layer thickness is on the order
of 10-1 nm, the effect of surface roughness of SiO2/Si on BSCO is much more limited.
Furthermore, BSCO monolayer has a BiO－SrO－CoO2－SrO－BiO structure (see Fig. 1A,
structure sketch), in which the inner CoO2 sublayer dominates the electric transport behavior and
is not in direct contact with SiO2/Si substrate. Therefore, the disorder potential due to the
underlying SiO2 surface is less important to the transport properties when compared to graphene
or MoS2.
References
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113527 (2007).
S2. P. Zhou, K.-A. Wang, P. C. Eklund, G. Dresselhaus, and M. S. Dresselhaus, Phys. Rev. B 48,
8412 (1993).
S3. C. H. Lui, L. Liu, K. F. Mak, G. W. Flynn, and T. F. Heinz, Nature 462, 339 (2009).
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