Enhanced Surface-and-Interface Coupling in Pd-Nanoparticle-coated LaAlO3/SrTiO3 Heterostructures: Strong Gas- and Photo-Induced Conductance Modulation

Pd nanoparticle (NP) coated LaAlO3/SrTiO3 (LAO/STO) heterointerface exhibits more notable conductance (G) change while varying the ambient gas (N2, H2/N2, and O2) and illuminating with UV light (wavelength: 365 nm) than a sample without the NPs. Simultaneous Kelvin probe force microscopy and transport measurements reveal close relationships between the surface work function (W) and G of the samples. Quantitative analyses suggest that a surface adsorption/desorption-mediated reaction and redox, resulting in a band-alignment modification and charge-transfer, could explain the gas- and photo-induced conductance modulation at the LAO/STO interface. Such surface-and-interface coupling enhanced by catalytic Pd NPs is a unique feature, quite distinct from conventional semiconductor hetero-junctions, which enables the significant conductance tunability at ultrathin oxide heterointerfaces by external stimuli.

T he LaAlO 3 /SrTiO 3 (LAO/STO) heterointerface, consisting of two wide-bandgap insulators, exhibits unexpected high-mobility two-dimensional electron gas (2DEG) behavior, which has generated an intense surge in research activities by the oxide electronics community  . Discovery of additional peculiar physical properties of the LAO/STO system, including a metal-insulator transition 2-5 , superconductivity 6 , magnetic ordering 7,8 , thermoelectricity 9 , strong electron correlation effect 10 , and a huge photoresponse [11][12][13][14][15] , has continued. Thus, investigations of these intriguing phenomena have surfaced as the most interesting and challenging topics in relevant fields. First, the electronic reconstruction was suggested as the origin of the unexpected 2Dconduction, which can avoid the polar catastrophe caused by residual net charges at the polar-LAO/nonpolar-STO interfaces 1,16,17 . Later, sophisticated structural and chemical analyses of the samples prepared at various growth conditions have led us to consider other possibilities, such as creation of oxygen vacancies in the STO substrate [18][19][20] , off-stoichiometry in the LAO layer 21 , interface doping by cation intermixing 22 , and defect generation during strain relaxation 23,24 .
Existence of a built-in electric field in LAO is evidence of the electronic reconstruction at the LAO/STO heterointerface 1 . Atomic force microscopy 3 , scanning tunneling spectroscopy 16 , and photocurrent 11,12 measurement results revealed a signature of the internal field. X-ray photoemission measurements, however, have not shown shifts or broadening of core-level spectra that would reflect the uncompensated internal field 25,26 . Such a discrepancy suggests that the aforementioned extrinsic effects -oxygen vacancies and cation disorder -at least partially contribute to the 2D conduction.
The LAO/STO heterointerface can be regarded as a capacitor: the insulating LAO layer is sandwiched by the top surface and the LAO/STO interface (and the underlying STO substrate) [27][28][29] . A nm-thick LAO film possesses large capacitance, and hence a slight change in the free/bound charges at the LAO surface should significantly modulate the carrier concentration at the LAO/STO interface. Surface charge modification, via biased AFM tip scanning 2-5 or polar molecule adsorption 30 concentration at the LAO/STO interface 11,32 . We should note that the conduction band alignment as well as the net charges at the surface and interface determines the potential drop across the LAO layer 27,28 . Adding a metal electrode on the surface of the LAO/STO heterostructure makes this point clear: the Schottky barrier height and band offset will affect the band alignment, subsequently influencing the interfacial carrier concentration 11,27,28 . All these considerations raise following interesting questions. ''How can we directly observe the surface-and-interface coupling of the LAO/STO heterointerface?'' ''Is it possible to enhance the surface-and-interface coupling?'' In this work, we demonstrate that simultaneous measurements of the surface potential and interface conductance of the LAO/STO system provide direct evidence of the strong surface-and-interface coupling. Also, we find that a LAO/STO sample coated with Pd nanoparticles (NPs) exhibits an enhanced relationship between the surface potential and interface conductance, compared with the bare sample. These results elucidate the underlying mechanism for remarkable ambient-dependent photoresponse of the Pd-NP-coated LAO/STO samples 15 . This work provides us with valuable insights regarding the 2DEG conduction at the LAO/STO heterointerface and further improvement of novel functional devices based on atomically controlled oxide heterostructures. Epitaxial LAO thin films with 5 unit cell thickness were grown on TiO 2 -terminated STO substrates by pulsed laser deposition (PLD) 15,31,33 . After the depositions, Pd NPs were coated on top of the LAO/STO surface using DC magnetron sputtering at room temperature. The NPs are crystallized and the density does not lead to a complete conduction percolation path, as shown in Fig. 1a. Detailed sample preparation procedures and characterization results can be found in earlier publications 15,33 . Hereafter the LAO/STO heterostructures without and with the Pd NPs will be called as LAO/STO and Pd/LAO/STO, respectively. Figure 1b schematically illustrates the measurement setup: transport and Kelvin probe force microscopy (KPFM) measurements were simultaneously performed in a glove box while varying the ambient gas environments 5 , such as H 2 (2%)/N 2 (98%), N 2 , and O 2 , and with and without UV light (wavelength: 365 nm and intensity: $10 mW/cm 2 ) illumination. Figure 2 shows conductance (G) and surface photovoltage (SPVdifference of work function before and after light illumination) of LAO/STO and Pd/LAO/STO as a function of time in different gases, obtained during gas exchange (from air to N 2 , from N 2 to H 2 /N 2 , and from N 2 to O 2 ) and illumination of UV light (detailed experimental procedures can be found in Method). In all the gases, G of the samples increases under the UV light. When the light is off, G decreases very slowly. Such a long relaxation time of the photocurrent, lasting over a day, is called persistent photoconductivity (PPC). PPC is believed to be a signature of trap states, mostly located near the surface and the interface of the samples 13,14 . Positive SPV (i.e., larger work function in dark) appears in all the cases. When the change in G is larger, larger SPV appears. SPV also shows long relaxation behavior like G. Such similar tendencies between G and SPV imply the existence of a common physical origin in these two quantities. It is also notable that the Pd NPs significantly enhance the gas sensitivity of the LAO/STO heterostructure, as reported earlier 33 . In particular, the gas exchange from N 2 to H 2 /N 2 induces a more abrupt conductance change of Pd/LAO/STO. First, we examine the ambient dependence. Figure 3a and 3b show the ambient dependence of G and work function (W) of the LAO/ STO and Pd/LAO/STO samples in dark, respectively. All the measured G Pd/LAO/STO values are smaller than the G LAO/STO values. As mentioned above, the LAO/STO sample can be regarded as a capacitor. Thus, the change in the potential drop across the LAO layer, V', should alter the sheet electron density at interface, s, where e LAO is the dielectric constant of LAO, e is the unit charge, and d LAO is the thickness of LAO 5,30 .
The large W of Pd (5.6 eV) causes band bending at the Pd/LAO interface 27,28 and the space charge region (SCR) is supposed to be wider than d LAO (only ,2 nm), as illustrated in Fig. 4a and 4b. An interfacial layer, which can be formed at two dissimilar metal and oxide layers, is included in Fig. 4a 34 . The Fermi level adjustment at the interface (i.e., band bending) by the Pd contact alters V' and consequently varies W (Fig. 4a). If the mobility (m) of both LAO/STO and Pd/LAO/STO is assumed to be almost the same at room temperature 18,30 , the relation between G and s can be obtained, From Eqs. 1 and 2, a following equation can be derived.
V' LAO/STO 2 V' Pd/LAO/STO is estimated to be 20.12 eV in N 2 , from the measured difference in the G values. As shown in the TEM image (Fig. 1a), the diameter of the Pd NPs and the distance between neighboring NPs are less than several nm, and hence KPFM cannot discern the work functions of the NPs and the bare LAO/STO surface. Thus, the measured surface work function of Pd/LAO/STO, W Pd/LAO/STO , corresponds to the weighted average of W Pd and W LAO/STO : We should note that G Pd/LAO/STO and W Pd/LAO/STO in H 2 /N 2 are largely different from those in N 2 and O 2 . Taking hydrogen adsorption into account, two possible processes, i.e., PdH x formation 36 and dissociate adsorption of H 2 on the LAO/STO surface 37 (Fig. 5a), can should be considered to explain DW, as illustrated in Fig. 5a. Potential drop by a surface dipole, Dw S , is given by a following equation where f is the surface coverage, N is the number of unit cells per area, d is the length of the dipole, e is the dielectric coefficient of the dipole layer 5 (Fig. 5b), V' and W increase, resulting in decrease of s (Eq. 1) 20 . The experimental data shows that the G of LAO/STO and Pd/LAO/STO decreases in O 2 compared to N 2 (Fig. 3a) As clearly shown in Fig. 3a and 3b, the gas response of LAO/STO is significantly enhanced by deposition of Pd NPs. The ambient    40,41 . Such a spillover effect contributes to the strong gas-dependent conductance modulation.
The UV-induced conductance change, DG light 5 G light 2 G dark , of the two samples in different gas (obtained from the data in Fig. 2) is compared in Fig. 6a. Figure 6b shows SPV, manifesting the close relationship between DG light and SPV. This reveals strong surfaceand-interface coupling of the LAO/STO heterointerface under UV illumination as well as in dark. DG light and SPV of Pd/LAO/STO exhibits a more notable ambient dependence, compared with LAO/STO. Research on gas sensor and photodetector applications have shown that the creation of photocarriers, charge transfer from/ to adsorbates, work function alteration of metal contacts by reaction, and surface band bending by trapping/detrapping of surface states can influence the bulk and contact resistance of oxide devices 33,35,40,41 . In most of these studies, dominant resistance modification occurs at the surface of oxides directly exposed to the external stimuli and just below the surface. In contrast, the resistance of the LAO/STO heterointerface is determined by the buried interface not the top surface. In this regard, the enhanced gas and UV sensitivity of the Pd/LAO/STO heterointerface is very distinct from those of other usual oxide systems and deserves more attention.
The incident photon energy (3.4 eV) is larger than the bandgap of STO (3.2 eV) but smaller than that of LAO (5.6 eV). Thus, these photons create electron-hole pairs in STO and the electrons move to the potential well at the LAO/STO interface, resulting in DG light . 0 ( of Fig. 7) [11][12][13][14] . The incident photons also can excite electrons from the Pd NPs, some of which can overcome the potential barrier at the Pd/LAO interface and produce a photocurrent ( of Fig. 7) 42 . The accumulated carriers at the LAO/STO interface decrease V' and W light , resulting in positive SPV (5W dark 2 W light ), as illustrated   in of Fig. 7. As discussed above, Pd NPs can cause upward bandbending in the LAO layer, by which electrons are quickly swept to the interface, increasing DG light and SPV. We did measure the mobility of our LAO/STO and Pd/LAO/STO samples before and after illumination of UV light by the Hall measurement system using the Van der Pauw configuration at room temperature. The mobility of the samples was almost the same. This suggests that the photo-response in the conductance should be attributed to the change in the carrier density.
In addition, the ambient dependence of Pd/LAO/STO is notable: the smallest DG light appears in H 2 /N 2 , compared with those in N 2 and O 2 . As aforementioned, small W of PdH x can reduce V' in H 2 containing gas and hence a low internal electric field in LAO would not readily separate photo-generated electron hole pairs, resulting in a weak UV response. The photoresponse in N 2 and O 2 also can be compared: DG light of Pd/LAO/STO in O 2 is smaller than that in N 2 . SPV of Pd/LAO/STO is almost the same in N 2 and O 2 . A decrease in oxygen vacancies will decrease DG light due to a smaller photocurrent contribution from the defect states [12][13][14]  ) and producing molecular oxygen (O 2 ) gas 33 . Such oxygen desorption decreases W light and a small W light increases SPV. Thus, the difference in N 2 and O 2 can be attributed to the photo-induced desorption. These results suggest that the catalytic activity of the Pd NPs plays an important role in ambient-dependent UV photoresponse of LAO/STO, further enhancing the surface-and-interface coupling.
Pd nanoparticle-coated LaAlO 3 /SrTiO 3 heterointerface (Pd/LAO/ STO) exhibits enhanced UV photoresponse compared with a bare sample (LAO/STO). The conductance and work function of Pd/ LAO/STO also exhibit a strong ambient dependence in N 2 , H 2 /N 2 , and O 2. Simultaneous measurements of conductance and surface work function in dark and UV illumination can reveal key physical processes involved in the ambient-and light-dependent response of Pd/LAO/STO samples: PdH x formation, oxygen vacancy annihilation, and surface adsorption/desorption with subsequent charge transfer. In particular, the quantitative analyses manifest the catalytic activity of Pd nanoparticles. These results demonstrate the unique features of the LAO/STO heterointerface, its strong surface and interface coupling and its crucial role in the intriguing transport phenomena.

Methods
The KPFM measurements were carried out using an atomic force microscopy system (XE-100, Park Systems Co.) with a glove box. Conductive Pt-coated Si cantilevers (NSG10/Pt, resonance frequency: ,240 kHz, NT-MDT) were used for both work function and topography measurements in the noncontact mode. Applied bias to the tip had both ac (frequency: v) and dc components. The lock-in technique allows extraction of the v component of the tip deflection, which is proportional to the electrostatic force between the tip and the sample. Just after each measurement, the work function of the tip was calibrated with the HOPG (highly ordered pyrolytic graphite, SPI Supplies) reference sample. Prior to the measurements, the samples were stored in dark for at least one day in air, whose relative humidity was 20-30%. The detailed experimental procedures for ambient-dependent KPFM measurements are described in our earlier publication (Ref. 5).
For the transport experiments, Al wires were bonded at the sample corner, where the top LAO layer was removed, as well as pads in a home-made PCB by a wire bonder (7476D, West Bond Inc.). The two-probe electrical measurements were performed by a semiconductor parameter analyzer (4156B, Hewlett Packard) simultaneously with the work function measurements in each gas.
The work function and conductance measurements were done in air at first, and then the glove box was purged by N 2 . After the purging for about 10 hours, the measurements were performed while maintaining the flow of N 2 gas. The identical measurements repeated in H 2 /N 2 gas in a similar way: ,10 hours of purging and continuing flow of H 2 /N 2 gas during the measurements. Prior to exchanging gas from H 2 /N 2 to O 2 , the system was purged by N 2 for more than 10 hours to remove residual H 2 gas and then filled with O 2 .
In order to examine photo-induced effects, we used a 10 mW UV light source with wavelength of 365 nm (SUV-S series, UVSMT) equipped with a power control unit. The UV light was delivered by a fiber to the sample and aligned to illuminate the center of the sample by a position-adjustable stand. The measurements in each gas were conducted with and without the UV light. Prior to the measurements, samples were stored in dark for at least one day, in order to exclude the possible influence of persistent photocurrent.