Starting solution. Hydrolysis reaction under thermostatic conditions. Check of viscosity and deposition test SOL. Deposition by spin coating

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1 Supplementary Figures Tetramethyl orthosilicate (TMOS) Tetrahydrofuran anhydrous (THF) Trimethyl methoxy silane (TMMS) Trimethyl silil acetate (TMSA) Starting solution Hydrolysis reaction under thermostatic conditions Dibutyl tin diacetate (DBTDA) H 2 O Poorly hydrolyzed SOL Check of viscosity and deposition test SOL Deposition by spin coating Sintering Supplementary Figure S1. Flow chart of the synthesis process. Flow chart of the synthesis process for the fabrication of nanostructured SnO 2 :SiO 2 thin films. 1

2 Supplementary Figure S2. Refractive index measurements of NS films. Prism-coupler measurements of guided modes at 633 nm in SnO 2 :SiO 2 sol-gel films at different steps of the synthesis process, with the calculated values of thickness and refractive index. (a) before heat treatment, (b) after treatment at 450 C, (c) after treatment at 1050 C. (d) Refractive index values at 633 nm in film with different Sn content and produced in different conditions. The error bars represent the standard error measured for at least 5 measurements per point. 2

3 Supplementary Figure S3. FTIR spectra for NS-films on silicon substrates. FTIR spectra for NSfilms on silicon substrates after the last stage of sintering. The films are produced starting from different concentrations of Sn-precursor and different oxygen partial pressure (see Supplementary Table 2 for details). 3

4 Supplementary Figure S4. Optical transmission of NS films on quartz. (a) UV Vis transmittance of SnO 2 :SiO 2 thin films with increasing Sn content (9, 15, 30%) deposited on fused quartz substrate and sintered in high (dashed lines) and low (full lines) partial pressure of oxygen. (b) Transmittance spectra after cleaning, compared with substrate spectrum (dotted line) of SnO 2 :SiO 2 thin films with increasing Sn content (9, 13%). 4

5 Supplementary Figure S5. Energy band diagram and current responses. Energy band diagram of a NS SnO 2 :SiO 2 thin film showing the top level of the valence band and the bottom level of the conduction band in the SiO 2 matrix (cyan), in the SnO 2 nanoparticles (blue), and in the SnO-like interphase at the nanoparticle surface (pink), as well as the band structure of p-si substrate (grey) and the gate Fermi level for the Ti electrode and Au capping layer. Possible charge transport paths for electrons (e - ) and holes (h + ) are indicated in the case of (a) forward and (b) reverse bias. Energy contributions from band bending, external electric field, and Coulomb repulsion in charged nanoparticles are not included. Note that hole-transport does occur only in forward bias (as emphasized by the grey arrows in b ) and is strictly enabled by the presence of the p-type SnO x<2 phase at the nanoparticle surface. (c) Current density characteristics of a UV NS-LED (nominal Sn concentration 16%). (d) Current density characteristics of a SnO 2 :SiO 2 film sintered in low oxygen partial pressure (nominal Sn concentration 15%).Inset: Electroluminescence spectra of a SnO 2 :SiO 2 film sintered in low oxygen partial pressure (nominal Sn concentration 15%). The signal at 0 V bias is reported as a black line for enabling the comparison between the signal-to-noise ratios. 5

6 Supplementary Figure S6. Charge transport in nanostructured films. lni vs. E 1/2 data in nanostructured films produced in different conditions and with different Sn content, with gate voltage in (a) reverse and (b) forward bias. Electroluminescent films show an almost linear dependence of lni with E 1/2 both in reverse and direct bias, showing a dominant Schottky behavior. Arrhenius plot (LnI vs. 1/kT ) of an electroluminescent (green circles) and a non-electroluminescent (orange circles) devices under (c) reverse and (d) forward bias. 6

7 Supplementary Tables Supplementary Table S1. Examples of sol composition for the synthesis of nanostructured SnO 2 - doped silica films. ml of reagent example1 example2 example3 example4 TMOS TMSA TMMS DBTDA THF At least 2.0 ml H 2 O

8 Supplementary Table S2. Summary of representative NS-Films synthesized with different initial amount of Sn precursor and in different conditions of sintering time and atmosphere. sintering Sn, % process Notes sinter. Conditions long sinter 2% O 2 in Ar S831 S813 S821 short sinter on Si-substrates on Si-substrates on transparent substrates 2% O 2 in Ar S % O 2 in Ar S831 S813b S821b S1022b 3.4% O 2 in Ar S 831r S1222 S813r S821r S 1212 S1032rb 0.3% O 2 in Ar S1032rb* 0.3% O 2 in Ar S1411 S1421 S1431 2% O 2 in Ar S1412 S1422 S1432 *Synthesized using a new batch of reagents with respect to sample S1022b in order to reduce the effects of aging of the reagents on the film quality. 8

9 Supplementary Discussion Charge Transport mechanism in SnO 2 :SiO 2 NS films The electrical properties of SnO 2 :SiO 2 NS films depend strongly on the nanoarchitecture of the material and in particular on the thickness of the substoichiometric interface between the SnO 2 nanocrystals and the silica matrix. The NP-host interphase, whose relative contribution to the nanophase depends on the synthesis parameters, is a SnO-like phase 36 with electric properties that differ from SnO 2. In fact, while SnO 2 is a wide-band gap semiconductor (E g = 3.6 ev) with n- type character due to oxygen vacancy defects (V O ), 38,52 SnO is a semiconductor with energy gap of 2.3 ev and p-type character 37. Therefore, the relative volume ratio between the n-type and the p- type domains in the film ultimately determines the percolation mechanism and the electroluminescence properties of the material. For this reason, accurate control of the synthesis and annealing parameter is required to produce NS films with balanced electron and hole transport features, so as to optimize the generation of excitons inside the film. Such control is achievable, for example, through suitable choice of annealing atmosphere. We may distinguish between two classes of devices: UV-LEDs with charge transport properties due to simultaneous injection and transport of both electrons and holes, resulting in efficient UV-EL; and devices with poor or none EL emission due to almost completely suppressed electron currents, which leads to strongly unbalanced electron/hole ratios. Such behaviours can be rationalized by looking at energy level diagrams in Supplementary Fig. S5a and S5b: in forward bias (negative bias applied at the cathode, Fig. S5a), holes are injected from the p-silicon substrate into the valence band of the SnO x<2 shell around the SnO 2 nanocrystals, and contribute to the electric current due to electrons injected from the cathode into the conduction band of SnO 2. When positive bias is instead applied at the cathode (reverse bias, Fig. S5b), charge transport can only be sustained by electrons injected from the p-si substrate into the conduction band of SnO 2, which leads to the lower currents registered in reverse bias. The simultaneous injection of both charge carries is crucial for achieving electroluminescent emission and, therefore, only NS films that allow hole and electron transport simultaneously are suitable as active layers in UV NS-LEDs (rappresentative IV response of a UV-LED is shown in Fig. S5c). Importantly, a particularly thick substoichiometric NC shell prevents injection of electrons into the 9

10 n-type SnO 2 cores. Accordingly, we register almost no currents in reverse bias in samples synthesized in conditions that induce thick oxygen substoichiometric interfaces (Supplementary Fig. S5d), such as thermal annealing in oxygen-poor atmosphere. In this case, the electric characteristic in forward bias is determined primarily by hole injection from the p-type Si substrate into the SnO-like interphase, which leads to an Ohmic I-V behaviour. Importantly, electron injection is strongly inhibited in films produced in conditions that promote the formation of thick substoichiometric interfaces (i.e. fast heating rates, low oxygen pressure). Hence, in devices incorporating this latter NS films, the relative concentration of holes and electrons is strongly unbalanced, which leads to weak electroluminescence emission (inset of Fig. S5d). Further, EL from such devices is limited to the red spectral region and no exciton-like contribution in the UV is observed, which suggests that charge transport is primarily sustained by defect states. Importantly, the injection process from the electrodes and the charge hopping within NPs requires non-null activation energies, which lead to thermally activated contributions to the conductivity. The electric response in reverse bias, as well as in forward bias in the case of electroluminescent films, reflects this kind of conduction regime with a typical Arrhenius behaviour. For electroluminescent devices, the electric response in both forward and reverse bias is not Ohmic and we observe a linear dependence of lni vs. E 1/2, suggesting that charge transport is mediated by a Schottky-like mechanism (Supplementary Figures S6a-b). In non-electroluminescent devices, the conduction in reverse bias is governed by p-type transport sustained by holes injected from the Si substrate and mediated by SnO-like shells around the SnO 2 NCs. In these cases, the fraction of the substoichiometric conductive path is relevant and results in a metal-like character of the NS film. Consequently, in reverse bias the thermal coefficient of resistivity may become positive (Fig. S6c-d). 10