Interplay of negative electronic compressibility and capacitance enhancement in lightly-doped metal oxide Bi0.95La0.05FeO3 by quantum capacitance model

Light-sensitive capacitance variation of Bi0.95La0.05FeO3 (BLFO) ceramics has been studied under violet to UV irradiation. The reversible capacitance enhancement up to 21% under 405 nm violet laser irradiation has been observed, suggesting a possible degree of freedom to dynamically control this in high dielectric materials for light-sensitive capacitance applications. By using ultraviolet photoemission spectroscopy (UPS), we show here that exposure of BLFO surfaces to UV light induces a counterintuitive shift of the O2p valence state to lower binding energy of up to 243 meV which is a direct signature of negative electronic compressibility (NEC). A decrease of BLFO electrical resistance agrees strongly with the UPS data suggesting the creation of a thin conductive layer on its insulating bulk under light irradiation. By exploiting the quantum capacitance model, we find that the negative quantum capacitance due to this NEC effect plays an important role in this capacitance enhancement

Bismuth Ferrite (BiFeO 3 ) is a multiferroic oxide material which has been extensively studied due to its ability to simultaneously exhibit both magnetic and strong ferroelectric properties at room temperature 1,2 . As such, BiFeO 3 has recently drawn much interest in potential applications spanning spintronics, magnetoelectric sensors and photovoltaic devices [3][4][5] . Several studies also attempt to enhance the dielectric constant of BiFeO 3 which could effectively improve its ferroelecticity 6 . In fact, a pure BiFeO 3 is difficult to synthesize whose dielectric constant was reported only 50-100 at 10 kHz 7,8 . Slight modifications to BiFeO 3 ceramics have been reported featuring both giant dielectric constant (>10 4 at room temperature) and sufficiently low dissipation factor 9,10 to be suitable for magnetodielectric applications 11 . Recently, efforts to improve the dielectric behavior of BiFeO 3 have been reported, such as varying preparation methods 12 and dopants 13 . Such extrinsic dielectric constant enhancement can be controlled by lattice distortions, particle sizes, domains, or impurities 12,14 .
In addition to extrinsic dielectric constant enhancement, the capacitance of oxide materials can intrinsically be improved by tuning carrier densities (i. e. n-type doping or applied gate electric field). Such dielectric tunabilities have potential applications as various microwave devices, such as phase shifters and varactors 15 . Recently, capacitance enhancement at the LaAlO 3 /SrTiO 3 interface in excess of 40% was found to originate from negative electron compressibility (NEC) at low electron density (n) 16 , enabled as the accumulation of all mobile electrons in the interfacial region which make quantum conduction and therefore quantum capacitance is the dominant model [16][17][18] . The negative thermodynamic density of state ( < μ 0 dn d ), where μ is chemical potential, has been observed in several 2D materials and interfaces 16,19 , carbon nanotubes 20 , and bulk materials 21  Alternatively, carrier densities on metal oxide surfaces can be controlled by the creation of oxygen vacancy states induced by light irradiation [22][23][24][25] . Recently, the capacitance enhancement induced by surface charge accumulation has been reported on CaCu 3 Ti 4 O 12 26 . This is similar to the case of applying electric field (i. e. introduction of the quantum conductive interfaces) which might indicate the analogous microscopic origin. In this work, we observe a striking capacitance enhancement in lightly-doped metal oxide Bi 0.95 La 0.05 FeO 3 (BLFO) under 405 nm violet laser irradiation. By using ultraviolet photoemission spectroscopy (UPS) and transport measurements, a signature of NEC has been revealed. By light irradiation, the experimentally-observed changes indicate that the quantum capacitance model plays a major role, which therefore supports claims for a strong interplay between NEC and capacitance enhancement. These findings are critical in understanding the fundamental nature of such system as well as establishing a new synthetic route to light-sensitive capacitive devices.

Methods
Sample preparation and characterisation. Our BLFO polycrystals were prepared by a simple co-precipitation method 9 . The dried precursors were calcined in air at 600 °C for 3 h. Sample powders were pressed into pellets with 1 cm diameter and sintered at 800 °C in air for 3 h. A small decrease of grain size (compared to the pure BiFeO 3 ) and structural distortion were revealed in BLFO samples by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The remarkable enhancement of BLFO capacitance might solely be affected by the reduction of electrical conductivity and leakage current 27 . We chose BLFO because of its high initial capacitance, for example, its dielectric constant was more than three times that of pure BiFeO 3 ceramics prepared by the same method 9,14,28 . Details of sample preparation and characterisation are shown in Supplementary Information. Capacitance measurement. Instead of a metal top electrode, our capacitor was fabricated with transparent conductive indium tin oxide (ITO) allowing light irradiation on this side. The BLFO sample was mechanically compressed against ITO and metal electrode as shown in Fig. 1(a). The capacitance measurements were performed using a standard impedance analyzer (Agilent: model 4294A) with alternating voltage output (V ac = 0.5 V) and frequency ranging from 1 kHz to 1 MHz. A 405 nm laser corresponding to 3.06 eV photon energy (with fixed intensity of 1.8 W.cm −2 and 3 × 3 mm 2 beamsize) was used throughout the experiment. This photon energy is smaller than the ITO optical band gap and work function of most materials (≈4eV), hence, competing effects occurring upon light irradiation such as light absorption and photoelectron ejection would not be expected 29 .
Ultraviolet photoemission spectroscopy. To understand the microscopic mechanism that drives this light-sensitive behavior in BLFO, the electronic structure of the BLFO sample was measured by ultraviolet photoemission spectroscopy (UPS) using Scienta R4000 electron analyzer located at BL 10.0.1 of the Advanced Light Source (USA) and BL 3.2a of the Synchrotron Light Research Institute (SLRI), Thailand. The measurements were performed at room temperature with base pressure better than 5 × 10 −8 mbar. Photon energy was set to be 60 eV with 0.3 × 0.1 mm 2 beam size.

Results and discussion
The BLFO capacitance measured at f = 2 kHz as a function of light irradiation is shown in Fig. 1(b). The initial capacitance measured in this setup is found to be around 200 pF. After irradiation for 240 s, the capacitance increased gradually and then became saturated at 236 pF. After turning off the irradiation, its capacitance decayed slowly and then reached the value close to the initial value. The frequency dependent measurements of capacitance and loss tangent before and after irradiation are shown in Fig. 1(c,d). The dielectric behaviour can be described by the Debye type equation including electrode and grain-boundary effects 30 .
Resistance measurement under light irradiation was performed on BLFO surfaces by preparing 1 mm-wide surface in between the gold electrodes ( Fig. 1(e)). After turning on the laser for 240 s, BLFO resistance decreased by 8%. Similar to the capacitance measurement, its resistance recovers close to the initial value after a 240 s absence of irradiation ( Fig. 1(f)). The features of the changes in capacitance and resistance suggest the existence of at least two effects occurring during the on-off process, including photogeneration of charge carriers (photoconductivity) and creation of oxygen vacancy. When light is on, photoconductivity is known to contribute to the changes in capacitance and resistance instantly and dynamically. It was found that both capacitance and resistance change quickly and immediately when turning off the light (at time ≈240 s of Fig. 1(b,f)) which could be described by the photogeneration of charge carriers. However, since the capacitance measured here does not instantly recover back to its original value when the irradiation is off, such changes are attributed to the creation of a thin conductive layer upon the insulating bulk referred to as a quantum-confined electron gas which is related to the creation of oxygen vacancies induced by light irradiation 26,31,32 .
The increases of capacitance and loss tangent are shown in Fig. 1(g,h). We found that the capacitance enhancement can be measured as high as 21% at frequency around 2 kHz after 240s of irradiation. The capacitance enhancement lineshape is nicely-fitted with incorporating of a reduction of surface resistivity (due to photogeneration of charge carrier) described by Maxwell-Wagner model 33 (see Supplementary Information) and NEC effect described below.
The UPS spectrum of the fresh sample was measured immediately (i. e. 0 min of irradiation, the black spectrum in Fig. 2(a)). It was found that the O 2p state located at a binding energy around 6-8 eV was consistent with other metal oxides 25,26,34 indicating a good electrical contact between our sample and a sample holder. A counterintuitive shift of the O 2p state to lower binding energy was initially observed by the first doping condition (5 min of UV light exposure) as illustrated by the red spectrum in Fig. 2(a). The UPS spectrum after zeroth order light irradiation, i. e., light with all frequencies, clearly indicates a significant shift of around 1 eV to lower binding energy (the blue spectrum in Fig. 2(a)). Moreover, an intergap peak emerged at around 4 eV binding energy which can be assigned to the oxygen vacancy state (V O ) 23,26 . In contrast, the C 1s state was also measured immediately after each UV irradiation (hν = 500 eV) which clearly indicates no binding energy shift of C 1s state (inset of Fig. 2(a)) confirming the distinctive character of BLFO sample. By using standard Gaussian fitting 35,36 , a continuous shift of O 2p state as a function of UV dosing was found to reach the value as high as 243 meV at a maximum UV dosing of 180 J.cm −2 as summarized in Fig. 2

(b) (see Supplementary Information).
Capacitance enhancement induced by light irradiation has been studied in several metal oxides 26,37 . To explain this, possible scenarios such as filling of material's mid-gap state 38 , enhancement of total charge carrier density 39 , and the creation of a two-dimensional electron gas at the surface 26,40 as a result of photo illumination have been proposed. Regarding these corroborated with our observed UPS spectra, we introduce the quantum capacitance (C q ) model to explain the capacitance enhancement in BLFO. This quantum capacitance model is a consequence of the Pauli principle which requires extra energy for filling a quantum well with electrons accumulated near the surface 18 .
In our case, C q formed on the irradiated-surfaces is manifested as capacitors in series with a geometric capacitance (C geo ) between two electrical plates 41 . Total capacitance (C tot ) can then be calculated by where C geo = A d ɛ is solely dependent on the geometry 16,42 . C q can be derived by the electron-electron interaction between layers (i. e. C q = C kin + C ex-corr ) which represents capacitances due to kinetic and exchange-correlation www.nature.com/scientificreports www.nature.com/scientificreports/ energies respectively. Notably, C q can be expressed by a term of thermodynamic density of states (C Ae q dn d 2 = μ ) 16 which strongly indicates that C q can either be positive or negative depending on the sign of μ dn d 16,43 . From above equation, C tot can be increased only in the case of negative C q which means < μ 0 dn d , whereby increasing the electron density leads to a decrease of chemical potential. In view of increasing C tot as a function of light irradiation (Fig. 1b), the absolute value of measured C q has been plotted by circle symbols in Fig. 3(a). Note that the similar values of C q estimated from UPS spectra and impedance measurement are shown in the inset of Fig. 3(a).
According to the available information of both NEC and quantum capacitance, the creation of two-dimensional electron density (n 2D corresponding to n) upon UV light exposure can be estimated by . 16 , where V e = μ is the shift of O 2p state and D is light dose (see Supplementary Information). As shown in Fig. 3(b), the calculated n 2D increases as a function of light dosing reaching the value of 0.95 × 10 10 cm −2 at 180 J.cm −2 . This exhibits similar trend with three orders of magnitude smaller than previous report of a light-irradiated surface of bulk SrTiO 3 (reproducing data is shown in the inset of Fig. 3(b) with permission from ref. 24 ). Regarding the expression of C q , μ dn d (or d dn μ ) is found to be a crucial parameter which offers a quantitative capacitance calculation. As shown in Fig. 3(c), the calculated d dn μ increases in negative values up to 43 × 10 −12 meV cm 2 at a maximum n 2D of 0.95 × 10 10 cm −2 . This value is about 2 times smaller than the previously observed 40% capacitance enhancement in LaAlO 3 /SrTiO 3 interfaces 16 . A plot of negative chemical potential shift versus n 2D is shown in Fig. 3(d). Note that the circle symbols are the measured data taken from Fig. 2(b). This negative value can be described by the random phase approximation (RPA) for exchange and correlation interactions where the negative compressibility (up to 600 meV) can happen in two-dimensional electron system 19,44 . Overall, we note that our observed NEC and the capacitance enhancement is generally described by the creation of two-dimensional electron layer on BLFO ceramic 16,45 induced by light irradiation.