For physical studies of correlated electron systems and for realizing novel device concepts, electrostatic modulation of metal-insulator transitions (MITs) is desired. The inherently high charge densities needed to modulate MITs make this difficult to achieve. The high capacitance of ionic liquids are attractive but, voltages are needed that can be in excess of the electrochemical stability of the system. Here, we show temperature/resistivity data that suggest electrostatic modulation of the MIT temperature of NdNiO3 in a wide regime. However, additional voltammetric and x-ray photoelectron spectroscopy measurements demonstrate the electrochemical impact of the electrostatic doping approach with ionic liquids.

Correlated electron systems such as neodymium nickelate (NdNiO3) exhibit metal insulator transitions (MITs) that are of fundamental physical interest and can potentially be used in advanced electronic devices.1–6 For MITs, where a band gap is created due to electron correlation, a transition to the insulating or metallic phase can be induced by a modulation of the charge carrier density. In NdNiO3, an MIT occurs between an antiferromagnetic insulating phase at low temperature and a paramagnetic metallic phase at high temperatures, after exceeding the MIT temperature (TMIT).

TMIT in NdNiO3 strongly depends on doping.7,8 It is typically necessary to change the electron concentration by at least 0.1 electron per unit cell to induce MIT phase transitions.9 The charge density can be influenced by extrinsic doping via impurities, intrinsic doping by stoichiometric changes, and electrostatic doping via charge injection. The former two methods cannot be achieved dynamically and for extrinsic doping lead to structural changes that add to the complexity of the phenomenon.7,10 Electrostatic doping does not rely on replacement or addition of atoms to the lattice. For example, electrostatic doping can be achieved by modulation doping at heterointerfaces,1 but does not allow for dynamic control of the charge density. Dynamic control is possible via the application of electrostatic fields by an external bias similar to the operation of field effect devices such as transistors.6,11–17,35

Here, we consider whether one can use electrostatic doping via an ionic liquid (IL) to achieve a sufficiently large change in carrier concentration in NdNiO3 to change the MIT. In order to account for 0.1 electrons per unit cell Δn ≈ 9 × 1020 cm–3 charges need to be induced in NdNiO3. Because electrostatic gating occurs at an interface, we must consider the capacitance required to achieve this carrier density. If the intrinsic Debye length of NdNiO3 is larger, or on the order of the film thickness, the doping via the surface charge density ρ can be calculated depending on the film thickness t. The thickness can also be expressed as the number of unit cells iuc with the idealized unit cell dimension zgr in growth direction. Nuc is the density of unit cells and f (>0.1) is the number of holes that should be induced per unit cell according to the following equation:

ρ=ftNuc|t=iuczgr.
(1)

For a layer of iuc = 7 unit cells (2.66 nm) grown along the b-axis, a doping of f = 0.1 holes per unit cell requires the areal charge density ρ = 2.4 × 1014 cm–2. Taking the electrical double layer capacitance of a widely used ionic liquid with the effective18 capacitance Ci = 5.54 μF/cm–2 as a basis a bias of about 6.9 V would have to be applied to induce 0.1 holes per unit cell. In practice, such high biases are well known to cause electrochemical reactions especially in the presence of oxygen at interfaces or if oxygenated species, O2 or H2O are present in the ionic liquid.6,20–23,32,33 In order to avoid the use of high biases, the thickness of the nickelate should be minimal. Although a thickness of less than 2.66 nm would reduce the required bias, reducing the thickness below 12 nm increasingly modifies the MIT due to strain effects mediated by the substrate.24 

While some have reported changes in electrical transport due to electrostatic gating by ionic liquids in correlated materials, we find that other effects must be considered as well. We show instead that we can achieve shifts in TMIT that are attributable to electrochemistry rather than electrostatics.25 We present transport data, voltammetry measurements, and surface analysis that expose the mechanism of the changes here in NdNiO3 films.

We studied the TMIT of NdNiO3 using gating by an IL with a process designed to minimize environmental contamination of the IL. Thin films (2.66 nm) of NdNiO3 were grown on NdGaO3 (110) substrates by RF magnetron sputtering at Ptot = 9 mTorr in an Ar/O2 atmosphere of 95%/5%. Fig. 1 shows the device mounted on a measurement chuck and a schematic of the electrode geometry. The samples for IL gating were prepared in a glove box with a background oxygen and water content of <1 ppm. A sealed polydimethylsiloxane (PDMS) cavity confines the IL and minimizes atmospheric contamination during transfer of the sample to a Quantum Design DynaCool Physical Property Measurement System (PPMS). On a NdGaO3 substrate, a stripe of NdNiO3 is contacted via two nickel/gold electrodes for resistance measurements and two sufficiently large gold working electrodes reach into the PDMS cavity to contact the IL 1,1- n- butyl- methylpyrrolidinium bis(trifluoromethanesulfonyl) azanide [BMPyr][TFSA] (TFSA aka. bis(trifluoromethylsulfonyl) imide [TFSI] or [NTf2], see Fig. 2). BMPyr was chosen as it is less acidic compared to other cations typical in ILs to provide less aggressive interface to the nickelate at negative biasing. We estimate that the PDMS protected sample was exposed to atmosphere for less than 1 min during loading and evacuation of the PPMS.

FIG. 1.

Left: Device on PPMS chuck. Right: Sketch of device design showing two contacts on patterned NdNiO3 and two externally joined cavity contacts extending into the sealed PDMS cavity without connecting to the NdNiO3 structure. The PDMS cavity is filled with an IL that can be biased via these cavity contacts.

FIG. 1.

Left: Device on PPMS chuck. Right: Sketch of device design showing two contacts on patterned NdNiO3 and two externally joined cavity contacts extending into the sealed PDMS cavity without connecting to the NdNiO3 structure. The PDMS cavity is filled with an IL that can be biased via these cavity contacts.

Close modal
FIG. 2.

Temperature profile of the NdNiO3 resistance at biases VG between −4 and +1 V. Inset: reversibility after negative gating. After VG = −3 V, the MIT temperature of 115 K at VG = 0 V cannot be reproduced. Lower-left: Molecular structure of [BMPyr][TFSA].

FIG. 2.

Temperature profile of the NdNiO3 resistance at biases VG between −4 and +1 V. Inset: reversibility after negative gating. After VG = −3 V, the MIT temperature of 115 K at VG = 0 V cannot be reproduced. Lower-left: Molecular structure of [BMPyr][TFSA].

Close modal

The temperature profile of the resistivity for different gate biases VG is shown in Fig. 2. To perform the measurement, VG is applied to the working electrodes at a temperature of T = 280 K, the temperature is swept to 23 K and back while the resistance of the NdNiO3 is measured. The timescale of the cooling step limits our ability to study any dynamics of the changes in conduction observed at low temperature compared to systems with higher transition temperatures.34 Between each bias VG ≠ 0, a measurement at VG = 0 V is recorded. If the measurement at 0 V showed a deviation of the resistance to the initial measurement, the measurement was continued with a new, but identical sample. The resistivity of the nickelate is calculated using the geometry of the NdNiO3 stripe, which is an acceptable approximation of the actual resistivity as confirmed with additional van der Pauw measurements without gating. The measurement shows the typical small hysteresis for very thin films around the TMIT of 115 K and a decreasing transition temperature and resistivity with increasing negative bias similar to results reported by Scherwitzl et al.26 

Examination of the electrical data shows an obvious degradation of the film's conductivity after a positive bias of just +1 V. For negative bias, the degradation is less obvious and is highlighted with the inset in Fig. 2. The plot shows TMIT in the order of the measurements and the MIT temperature of the run at zero gate bias, recorded after −2.8 and −3 V respectively. Up to TMIT = 113 K at VG = −2.8 V, the measurement is absolutely reversible and the changes in MIT temperature of −2 K are small. At VG = −3 V, where about 4.4% hole doping is induced, the IL gating starts to alter the layer of NdNiO3 permanently. More dramatic electrochemical changes between the initial run at VG = 0 V and the zero gate run after a bias of just VG = −2.5 V were reported by Asanuma et al.27 with ΔTMIT = 13 K. In their work, they show a 5 nm film of NdNiO3 biased with a more reactive IL can exhibit changes of ΔTMIT = −40 K at VG = −2.5 V.

To gain more insight into the origin of changes in TMIT, we examined the electrical behavior of the NdNiO3 films at 294 K by voltammetry and gated conductivity (“field effect”) measurements. The results for application of negative and positive bias (versus gold) are shown in Figs. 3(a) and 3(b), respectively. Voltammetric measurements with a gate sweep ranging over −3 V, Fig. 3(a) show a small reduction peak of the NdNiO3 at −3 V accompanied by the appearance of additional peaks at −2.2 V and −0.6 V that stem from the reactions with the products. The gated sheet resistance measurement, shown in the inset of Fig. 3(a), shows a strongly decreasing differential sheet conductance that starts at voltages more negative than −2.5 V and points at an effect superimposed to the electrostatic field-effect.

FIG. 3.

Cyclo voltammetry measurements for (a) the negative and (b) the positive regime versus Au working electrodes with contact area bigger than the NdNiO3 channel area. The different colors from dark (black) to light (yellow) show the sweep range increasing from |1.5| to |4|V successively. The insets show the bias dependent sheet conductance measured with a 50 mV source/drain test voltage, briefly applied at maximum (minimum) gate bias during the measurements.

FIG. 3.

Cyclo voltammetry measurements for (a) the negative and (b) the positive regime versus Au working electrodes with contact area bigger than the NdNiO3 channel area. The different colors from dark (black) to light (yellow) show the sweep range increasing from |1.5| to |4|V successively. The insets show the bias dependent sheet conductance measured with a 50 mV source/drain test voltage, briefly applied at maximum (minimum) gate bias during the measurements.

Close modal

Looking at the positive bias regime in Fig. 3(b) reveals an even more dramatic effect. The voltammetry measurements show a distinct oxidation peak at +1.25 V accompanied with a dramatic drop in conductivity, likewise illustrated by the steep decay of the sheet conductance down to the measurement resolution of 10−12 Siemens · Sq. The electrochemical oxidation of the NdNiO3 (and reduction of Ni+3 to Ni+2, vide infra) in this regime starts at small voltages and rapidly leads to the destruction of the layer. These measurements reveal the onset of substantial change in the NdNiO3 layer within the timescale of the voltage sweep, but it is important to realize that these processes also occur at slower rates at lower voltages as well. At the given number of unit cells in an 1.5 mm2 NdNiO3 stripe (3.6 × 1013 unit cells), the whole film would disintegrate in 1 s at a current of 5.7 × 10−6 A, considering one reactive event per charge. To change 0.1 electrons/unit cell in the NdNiO3 surface even 8 × 10−8 A for 1 s is sufficient.

We analyzed the nature of the irreversible electrochemical changes when the nickelate is subjected to IL gating via X-ray photoelectron spectroscopy (XPS). Figs. 4(a)–4(c) show the scans of the valence band, the oxygen 1 s, and the nickel 2 p energies after the devices were biased at +1.0 V, 0.0 V, or −4.0 V for 5 min and thoroughly rinsed in acetronitrile to remove most of the IL. The scan of the valence band in Fig. 4(a) shows the erosion of states near the Fermi level after biasing with +1 V. The electrochemical oxidation of the film leads to the formation of an insulator. For the negative biasing however, Fig. 4(a) does not show any significant change compared to the VG = 0 V scan.

FIG. 4.

XPS at the (a) valence, (b) oxygen, and (c) nickel edge of NdNiO3 after biasing for 5 min and rinsing off the ionic liquid.

FIG. 4.

XPS at the (a) valence, (b) oxygen, and (c) nickel edge of NdNiO3 after biasing for 5 min and rinsing off the ionic liquid.

Close modal

Table I shows the peak positions of the oxygen 1 s emission shown in Fig. 4(b). The higher binding energy around 532.3 eV at position i is present in all samples and stems from decomposition products of the ionic liquid and other contaminants at the surface19,28 (see the supplementary material for carbon 1 s spectra). The lower binding energy around 528.8 eV at position iii can be attributed to oxygen atoms bound in the perovskite structure around Ni3+.29 While this peak disappears for the positive bias, it is slightly shifted to higher energies after negative biasing. The O1s binding energy around 530 eV at position ii can be ascribed to surface oxygen in the NdNiO3 crystal with a broken bond to the surface or to stoichiometries, where Ni is bivalently bound to oxygen (cf. Fig. 3(b) cathodic peak and reduction of Ni+3 to Ni+2). This peak also decreases for the negatively biased sample, while it becomes the dominant contribution after positive biasing.

TABLE I.

Oxygen 1s binding energies

Biasi (eV) (%)ii (eV) (%)iii (eV) (%)
+1 V 532.21 (53) 529.85 (47) – (0) 
0 V 532.35 (72) 530.01 (15) 528.76 (13) 
–4 V 532.30 (83) 530.14 (5) 529.06 (12) 
Biasi (eV) (%)ii (eV) (%)iii (eV) (%)
+1 V 532.21 (53) 529.85 (47) – (0) 
0 V 532.35 (72) 530.01 (15) 528.76 (13) 
–4 V 532.30 (83) 530.14 (5) 529.06 (12) 

Whereas it seems obvious that the perovskite is modified chemically after positive bias, the effects of the negative bias are less clear. Compared to the unaltered sample, the perovskite oxygen peak at position iii is shifted by 0.3 eV to higher energies, hinting at decrease of charge concentration at the oxygen atoms. This observation is consistent with the measured decrease in conductivity but does not reveal a specific chemical modification. However, the 30% decrease in the surface signal at position ii (cf. Table I) leads to the assumption that the oxygen terminated crystal surface holds fewer oxygen atoms, hence, the IL has etched the surface. In that case the binding energy of Ni 2 p 3/2 should reveal a higher contribution of Ni+ around 853.5 eV in relation to Ni3+ around 856.5 eV,30 but the Ni 2 p spectra in Fig. 4(c) do not support this hypothesis. In view of the increase at position i in Fig. 4(b), it cannot be ruled out that the negative bias produces a contribution at 531.8 eV according to the formation of Ni2O3. However, it should be stated that a surface charge density of 1014 cm−2 would require less than 30% of the surface oxygen atoms to lose an electron so that even small electrochemical changes to the NdNiO3 surface already have at least the same effect as electrostatic doping would induce.

It can be concluded that the temperature/resistivity characteristics shown in Fig. 2 are not a result of purely electrostatic doping. Regarding the voltammetric analysis, bias voltages in excess of VG ≤ −3 V and VG ≥ +1 V clearly lead to electrochemical reactions, similar to reports for VO231 and SrTiO3.32 However, a small change in the surface atoms in the thin NdNiO3 layer can achieve surface charge densities comparable to what can be achieved with an electrical double layer. Hence, even at small biases, below the critical voltages, the nickelate can be modified electrochemically and likely reversibly. It is therefore difficult to isolate changes due to purely electrostatic effects in this system. There are clearly changes in the electrical properties of the material that are indeed reversible using IL gating under the conditions studied here. If such electrochemical effects can be harnessed and the changes in the material system are well understood, we expect that they can provide a window into interesting electronic properties as has previously been studied in other systems.32,35

This work was provided by the MRSEC Program of the NSF under Award No. DMR-1121053. The work also made use of the UCSB Nanofabrication Facility, a part of the NSF–funded NNIN network. S.B. was funded by the Deutsche Forschnungsgemeinschaft (DFG) Project No. BU 2669/1-1 and A.J.H. by an Elings Prize Fellowship of the California Nanosystems Institute at University of California, Santa Barbara.

1.
J.
Son
,
S.
Rajan
,
S.
Stemmer
, and
S. J.
Allen
,
J. Appl. Phys.
110
,
084503
(
2011
).
2.
M. D.
Pickett
,
G.
Medeiros-Ribeiro
, and
R. S.
Williams
,
Nat. Mater.
12
,
114
(
2013
).
3.
A.
Zimmers
,
L.
Aigouy
,
M.
Mortier
,
A.
Sharoni
,
S.
Wang
,
K. G.
West
,
J. G.
Ramirez
, and
I. K.
Schuller
,
Phys. Rev. Lett.
110
,
056601
(
2013
).
4.
L.
Pellegrino
,
N.
Manca
,
T.
Kanki
,
H.
Tanaka
,
M.
Biasotti
,
E.
Bellingeri
,
A. S.
Siri
, and
D.
Marre
,
Adv. Mater.
24
,
2929
(
2012
).
5.
T.
Driscoll
,
H. T.
Kim
,
B. G.
Chae
,
M.
Di Ventra
, and
D. N.
Basov
,
Appl. Phys. Lett.
95
,
043503
(
2009
).
6.
Y.
Zhou
and
S.
Ramanathan
,
Crit. Rev. Solid State Mater. Sci.
38
,
286
(
2013
).
7.
J. L.
Garcia-Munoz
,
M.
Suaaidi
,
M. J.
Martinezlope
, and
J. A.
Alonso
,
Phys. Rev. B
52
,
13563
(
1995
).
8.
S. W.
Cheong
,
H. Y.
Hwang
,
B.
Batlogg
,
A. S.
Cooper
, and
P. C.
Canfield
,
Physica B
194
,
1087
(
1994
).
9.
C. H.
Ahn
,
A.
Bhattacharya
,
M.
Di Ventra
,
J. N.
Eckstein
,
C. D.
Frisbie
,
M. E.
Gershenson
,
A. M.
Goldman
,
I. H.
Inoue
,
J.
Mannhart
,
A. J.
Millis
,
A. F.
Morpurgo
,
D.
Natelson
, and
J.-M.
Triscone
,
Rev. Mod. Phys.
78
,
1185
(
2006
).
10.
J. A.
Alonso
,
M. J.
Martinezlope
, and
M. A.
Hidalgo
,
J. Solid State Chem.
116
,
146
(
1995
).
11.
A.
Cassinese
,
G. M.
De Luca
,
A.
Gambardella
,
A.
Prigiobbo
,
M.
Salluzzo
, and
R.
Vaglio
,
IEEE Trans. Appl. Supercond.
15
,
2946
(
2005
).
12.
Y.
Lee
,
C.
Clement
,
J.
Hellerstedt
,
J.
Kinney
,
L.
Kinnischtzke
,
X.
Leng
,
S. D.
Snyder
, and
A. M.
Goldman
,
Phys. Rev. Lett.
106
,
136809
(
2011
).
13.
M.
Li
,
T.
Graf
,
T. D.
Schladt
,
X.
Jiang
, and
S. S. P.
Parkin
,
Phys. Rev. Lett.
109
,
196803
(
2012
).
14.
S. D.
Ha
,
U.
Vetter
,
J.
Shi
, and
S.
Ramanathan
,
Appl. Phys. Lett.
102
,
183102
(
2013
).
15.
S.
Bubel
,
S.
Meyer
,
F.
Kunze
, and
M. L.
Chabinyc
,
Appl. Phys. Lett.
103
,
152102
(
2013
).
16.
T.
Katase
,
H.
Hiramatsu
,
T.
Kamiya
, and
H.
Hosono
,
Proc. Natl. Acad. Sci. U.S.A.
111
,
3979
(
2014
).
17.
H. T.
Yi
,
B.
Gao
,
W.
Xie
,
S.-W.
Cheong
, and
V.
Podzorov
,
Sci. Rep.
4
,
6604
(
2014
).
18.
H.
Yuan
,
H.
Shimotani
,
J.
Ye
,
S.
Yoon
,
H.
Aliah
,
A.
Tsukazaki
,
M.
Kawasaki
, and
Y.
Iwasa
,
J. Am. Chem. Soc.
132
,
18402
(
2010
).
19.
See supplementary material at http://dx.doi.org/10.1063/1.4915269 for impedance spectroscopy of [BMPyr][TFSA], XPS C1s spectra, and experimental procedures.
20.
A. M.
O'Mahony
,
D. S.
Silvester
,
L.
Aldous
,
C.
Hardacre
, and
R. G.
Compton
,
J. Chem. Eng. Data
53
,
2884
(
2008
).
21.
K.
Ueno
,
H.
Shimotani
,
Y.
Iwasa
, and
M.
Kawasaki
,
Appl. Phys. Lett.
96
,
252107
(
2010
).
22.
S.
Bubel
,
S.
Meyer
, and
M. L.
Chabinyc
,
IEEE Trans. Electron Devices
61
,
1561
(
2014
).
23.
A. C.
Lang
,
J. D.
Sloppy
,
H.
Ghassemi
,
R. C.
Devlin
,
R. J.
Sichel-Tissot
,
J.-C.
Idrobo
,
S. J.
Ma
, and
M. L.
Taheri
,
ACS Appl. Mater. Inter.
6
,
17018
(
2014
).
24.
J.
Liu
,
M.
Kareev
,
B.
Gray
,
J. W.
Kim
,
P.
Ryan
,
B.
Dabrowski
,
J. W.
Freeland
, and
J.
Chakhalian
,
Appl. Phys. Lett.
96
,
233110
(
2010
).
25.
H.
Ji
,
J.
Wei
, and
D.
Natelson
,
Nano Lett.
12
,
2988
(
2012
).
26.
R.
Scherwitzl
,
P.
Zubko
,
I. G.
Lezama
,
S.
Ono
,
A. F.
Morpurgo
,
G.
Catalan
, and
J.-M.
Triscone
,
Adv. Mater.
22
,
5517
(
2010
).
27.
S.
Asanuma
,
P. H.
Xiang
,
H.
Yamada
,
H.
Sato
,
I. H.
Inoue
,
H.
Akoh
,
A.
Sawa
,
K.
Ueno
,
H.
Shimotani
,
H.
Yuan
,
M.
Kawasaki
, and
Y.
Iwasa
,
Appl. Phys. Lett.
97
,
142110
(
2010
).
28.
K.
Galicka
,
J.
Szade
,
P.
Ruello
,
P.
Laffez
, and
A.
Ratuszna
,
Appl. Surf. Sci.
255
,
4355
(
2009
).
29.
R.
Gottschall
,
R.
Schollhorn
,
M.
Muhler
,
N.
Jansen
,
D.
Walcher
, and
P.
Gutlich
,
Inorg. Chem.
37
,
1513
(
1998
).
30.
T.
Moriga
,
O.
Usaka
,
T.
Imamura
,
I.
Nakabayashi
,
I.
Matsubara
,
T.
Kinouchi
,
S.
Kikkawa
, and
F.
Kanamaru
,
Bull. Chem. Soc. Jpn.
67
,
687
(
1994
).
31.
J.
Jeong
,
N.
Aetukuri
,
T.
Graf
,
T. D.
Schladt
,
M. G.
Samant
, and
S. S. P.
Parkin
,
Science
339
,
1402
(
2013
).
32.
M.
Li
,
W.
Han
,
X.
Jiang
,
J.
Jeong
,
M. G.
Samant
, and
S. S. P.
Parkin
,
Nano Lett.
13
,
4675
(
2013
).
33.
M. D.
Goldflam
,
M. K.
Liu
,
B. C.
Chapler
,
H. T.
Stinson
,
A. J.
Sternbach
,
A. S.
McLeod
,
J. D.
Zhang
,
K.
Geng
,
M.
Royal
,
B.-J.
Kim
,
R. D.
Averitt
,
N. M.
Jokerst
,
D. R.
Smith
,
H. T.
Kim
, and
D. N.
Basov
,
Appl. Phys. Lett.
105
,
041117
(
2014
).
34.
J.
Jeong
,
N. B.
Aetukuri
,
D.
Passarello
,
S. D.
Conradson
,
M. G.
Samant
, and
S. S. P.
Parkin
,
Proc. Natl. Acad. Sci. U.S.A.
112
,
1013
(
2015
).
35.
J.
Shi
,
S. D.
Ha
,
Y.
Zhou
,
F.
Schoofs
, and
S.
Ramanathan
,
Nat. Commun.
4
,
2676
(
2013
).

Supplementary Material