Tunable near-infrared and visible-light transmittance in nanocrystal-in-glass composites

Amorphous metal oxides are useful in optical, electronic and electrochemical devices. The bonding arrangement within these glasses largely determines their properties, yet it remains a challenge to manipulate their structures in a controlled manner. Recently, we developed synthetic protocols for incorporating nanocrystals that are covalently bonded into amorphous materials. This ‘nanocrystal-in-glass’ approach not only combines two functional components in one material, but also the covalent link enables us to manipulate the glass structure to change its properties. Here we illustrate the power of this approach by introducing tin-doped indium oxide nanocrystals into niobium oxide glass (NbOx), and realize a new amorphous structure as a consequence of linking it to the nanocrystals. The resulting material demonstrates a previously unrealized optical switching behaviour that will enable the dynamic control of solar radiation transmittance through windows. These transparent films can block near-infrared and visible light selectively and independently by varying the applied electrochemical voltage over a range of 2.5 volts. We also show that the reconstructed NbOx glass has superior properties—its optical contrast is enhanced fivefold and it has excellent electrochemical stability, with 96 per cent of charge capacity retained after 2,000 cycles.

Amorphous metal oxides are useful in optical 1,2 , electronic [3][4][5] and electrochemical devices 6,7 . The bonding arrangement within these glasses largely determines their properties, yet it remains a challenge to manipulate their structures in a controlled manner. Recently, we developed synthetic protocols for incorporating nanocrystals that are covalently bonded into amorphous materials 8,9 . This 'nanocrystalin-glass' approach not only combines two functional components in one material, but also the covalent link enables us to manipulate the glass structure to change its properties. Here we illustrate the power of this approach by introducing tin-doped indium oxide nanocrystals into niobium oxide glass (NbO x ), and realize a new amorphous structure as a consequence of linking it to the nanocrystals. The resulting material demonstrates a previously unrealized optical switching behaviour that will enable the dynamic control of solar radiation transmittance through windows. These transparent films can block near-infrared and visible light selectively and independently by varying the applied electrochemical voltage over a range of 2.5 volts. We also show that the reconstructed NbO x glass has superior properties-its optical contrast is enhanced fivefold and it has excellent electrochemical stability, with 96 per cent of charge capacity retained after 2,000 cycles.
Enhanced properties have been demonstrated when nanocrystalline precipitates form within a glassy matrix [10][11][12][13][14][15] . These crystalline domains can harden bulk glasses 10 , introduce optical nonlinearity 11 or lead to anomalous transparency 12 . Anecdotal evidence also suggests that ion transport may be enhanced at crystal-glass interfaces. For instance, when LiAlSiO 4 was partially crystallized, fast relaxation of ions was observed 13 , whereas precipitating TiO 2 nanocrystals within mesoporous P 2 O 5 glass led to high-power lithium (Li) ion battery electrodes 14 . Structural modifications at crystal-glass interfaces have been proposed to explain such observations, but no direct evidence has been reported owing to the limited structural and compositional control offered by those preparation methods. Conventional methods of preparing glass ceramics rely on thermally induced phase separation and in situ crystallization processes, which are very complex to control experimentally 16 . The timetemperature parameter space for selective nanocrystallization is typically narrow and strongly influenced by the glass composition, structure and thermal history 16 . Therefore, determining the heat-treatment conditions required for selective crystallization of a desired composition phase with high control over the interface area becomes not only a challenging and tedious task, but also a nearly impossible task for complex multicomponent stoichiometries (for example, doped nanocrystals). Here we show how to control these characteristics by using well-defined colloidal nanocrystals that are post-synthetically embedded in and chemically linked to glass.
In our colloidal approach, chemical bonds are made by combining ligand-stripped Sn-doped In 2 O 3 (ITO) nanocrystals 17,18 and polyniobate clusters (POMs) in solution 9 . This process results in POM-stabilized nanocrystal dispersions (Fig. 1a, Supplementary Fig. 2), from which films are deposited and then thermally annealed in air, at T 5 400 uC (Fig. 1b). X-ray diffraction (XRD) analysis of as-deposited and annealed films shows that the crystalline molecular POMs condense to an amorphous NbO x matrix ( Supplementary Fig. 3). In addition, XRD patterns (Fig. 1c, d) and scanning transmission electron microscopy cross-sectional images (Fig. 1e, f) confirm the amorphous nature of the NbO x matrix and that the embedded ITO nanocrystals remain highly crystalline. This synthetic approach is very flexible, permitting the selection of unusual combinations of components to tailor functionality. For instance, ITO nanocrystals and amorphous NbO x were specifically selected for their spectrally distinct electrochromic response. The volume fraction was finely tuned over a broad range, from 0% to 69%, by simply adding additional POM to the aqueous POM-stabilized nanocrystal dispersion (Fig. 1a). This tunability is key to revealing the reconstruction of the NbO x glass structure induced by its covalent linkage to ITO nanocrystals.
To evaluate the structure of the glass, the Raman spectrum of a pure amorphous NbO x film was compared to those of ITO-in-NbO x with different nanocrystal-glass interfacial areas (Fig. 2 Intensity (a.u.)   9 . c, Two-dimensional XRD image of a ITO-in-NbO x film. d, XRD pattern obtained after integrating along the out-of-plane scattering direction Q z . The indexed diffraction lines result from randomly oriented ITO nanocrystals (cubic In 2 O 3 bixbyite structure) whereas the broad peak at Q 5 1.85 Å 21 is assigned to amorphous NbO x . e, f, Low-and high-resolution cross-sectional scanning transmission electron microscopy Z-contrast images (see Methods). are readily observed in the range 500 cm 21 to 1,000 cm 21 and changes in their peak positions and relative intensities track with the interfacial area. This information was used to identify the structural changes in the glass as a result of linking to the nanocrystals.
The topological structure of NbO x glass is characterized by how the [NbO 6 ] octahedral units are connected, namely through vertices and along edges, as well as by the degree of connectivity in the disordered network. Given the higher electrostatic repulsions in edge-sharing topologies compared to vertex-sharing 19 , distinctive vibrational modes for the bridging oxygens can be observed 20 . By deconvolving these bridging oxygen bands (550 cm 21 to 750 cm 21 ), we find that highly distorted edge-sharing [NbO 6 ] are prevalent in the nanocrystal-in-glass films, whereas pure NbO x contains more regular, vertex-sharing arrangements (Fig. 2, Table 1, Supplementary Fig. 4). In addition, the relative intensity of Raman peaks at higher frequencies (750 cm 21 to 950 cm 21 ) increases at higher nanocrystal loading, suggesting reduced connectivity of the glassy network. In particular, the highest-frequency peak, 900 cm 21 to 925 cm 21 , is assigned to terminal Nb-O bonds. The ratio between these terminal bonds and bridging ones is greater in the nanocomposites (Fig. 2, Table 1, Supplementary Fig. 4), which we attribute to internal interruption of the polymerized network. Generally, the degree of connectivity in oxide glasses can be reduced by addition of network modifiers or glass intermediates 21 . In our case, we suggest that indium (In) and tin (Sn) from the ITO nanocrystals diffuse locally into the NbO x glass during thermal processing. There they disrupt the glassy network, inducing chain breaking and thus reducing the network connectivity. Indeed, XRD patterns of films processed at increasing temperatures (400 uC-550 uC) showed broader ITO peaks (Supplementary Fig. 5b), pointing to progressive interfacial dissolution of ITO in the NbO x matrix. The presence of a ternary InNbO 4 phase following high-temperature annealing (.550 uC) (Supplementary Fig. 5c, d) further supports the idea that In and probably Sn diffuse into the NbO x matrix adjacent to the nanocrystals. Hence, the structure of the NbO x glass is profoundly altered by covalent linking to ITO nanocrystals, becoming highly distorted, less interconnected and interfacially doped (Fig. 3).
The consequence of linking ITO nanocrystals within a NbO x glass matrix is that a unique electrochromic optical switching functionality can be realized (Fig. 4). As explained below, this new optical behaviour not only reflects the individual properties of the two components, but also is quantifiably superior as a result of the reconstruction of the amorphous material.
Amorphous metal oxides are leading candidates for dynamically controlling solar radiation transmitted through 'smart' windows by means of their electrochromic properties. Electrochromism is a reversible change of optical transmittance in response to electrochemical charging and discharging. After more than 40 years of research in electrochromics 22 , spectral selectivity, that is, independent modulation of visible and near-infrared (NIR) radiation, is still considered a 'holy grail' for reducing the energy needed to light and thermally regulate building interiors. For instance, others have proposed combining thermochromic and electrochromic materials to modulate NIR and visible light, respectively 23 , although fully integrated devices based on this concept have yet to be reported. We took a step towards this goal through our recent demonstration that charged ITO nanocrystals selectively block NIR light through a plasmonic electrochromic effect 24 (Fig. 4f). In contrast, amorphous transition metal oxides such as NbO x modulate mostly visible light under electrochemical bias (Fig. 4a). Now, by linking these two materials and controlling the applied voltage, we find that the transmittance can be modulated in both spectral bands ( Fig. 4a-g, Supplementary Fig. 1). The nanocrystal-in-glass coating switches progressively between three optical states: fully transparent (at 4 V versus Li), selectively NIR blocking (around 2.3 V) and broadband blocking of visible and NIR (1.5 V). Thus, solar radiation can now be dynamically modulated with spectral selectivity ( Supplementary  Fig. 1). The modulation of NIR light derives from the free electrons in the ITO, whose concentration reaches approximately 1.9 3 10 21 cm 23 at 2.3 V ( Supplementary Information and Supplementary Fig. 9). Ultimately, the modular construction of our materials will facilitate even greater modulation of NIR transmittance as new plasmonic nanocrystals are developed with a higher free-electron concentration and a broader dynamic optical range 25 .
Remarkably, the modest visible-light modulation of amorphous NbO x (Fig. 4a) was greatly exceeded by that of the nanocrystal-in-glass composites ( Fig. 4b-d), indicating that the reconstructed NbO x glass is far more electrochemically active. The optical contrast in the visible range monotonically increased with nanocrystal content up to ,43% ITO,

RESEARCH LETTER
reaching a value that is five times greater than that of pure NbO x (Fig. 4h). At higher nanocrystal loading, the contrast decreased again, indicating that the enhancement is maximized when the average nanocrystal spacing is around 0.7 nm (Supplementary Information). At this high interface density (0.54 nm 21 ), the structure of the NbO x is rearranged throughout, effectively resulting in a new polymorph that exhibits functional characteristics distinct from those of the bulk glass, in fact resembling more the stronger electrochromism seen in the high-temperature pseudohexagonal crystalline phase of Nb 2 O 5 (known as the TT-phase) (ref. 26). The reconstructed, nanocrystal-linked amorphous NbO x network is also highly stable to insertion and extraction of Li ions. The electrochemical cycling stability exceeds that of either individual component; only 4% charge capacity was lost after 2,000 cycles (between 4 V and 1.5 V), whereas pure NbO x and especially ITO nanocrystal films degraded substantially (20% and 85% capacity losses, respectively) (Supplementary Fig. 7a). In agreement with the electrochemical stability of ITO-in-NbO x composites, the dual-band optical modulation characteristics were maintained after cycling (Supplementary Fig. 7b). We suggest that the enhancements in both optical contrast and stability are related to the more open network structure of the reconstructed glass, which facilitates ion insertion and extraction and helps to relax the stress induced by these processes. Consistent with this hypothesis, the charge capacity increases proportionally with the optical contrast (Supplementary Fig. 8).
Linking undoped In 2 O 3 nanocrystals to NbO x , instead of Sn-doped In 2 O 3 nanocrystals, similarly enhanced the optical contrast arising from the matrix (Fig. 4h, Supplementary Fig. 10), which eliminates the possibility that potential electrochemically active Sn species could play a part in such enhancement. Instead, this result further confirms that the optical contrast enhancement is related to a structural reconstruction of the NbO x matrix as a consequence of the covalent linkage to the embedded nanocrystals. Very probably, similar rearrangements of the glass structure could be involved in the enhanced ionic transport properties previously reported 13,14,27 in diverse nanocrystal-in-glass materials made by a variety of methods. Therefore, reconstructing glass In the inset, the niobium atoms are located at the centre of the green octahedra, oxygen atoms are red and indium atoms are blue.

LETTER RESEARCH
via nanocrystal linking could represent a general strategy for the manipulation of ion transport. In essence, this nanocrystal-in-glass approach represents the next step in the evolution of the concept of linking building units together to construct new functional materials. Analogously, organic molecules are held together by weak, dynamic bonds to form supramolecular assemblies 28 , metal ions are covalently bonded to organic struts to form metal-organic frameworks 29 and nanocrystals are linked directly to each other in mesoporous architectures 30 . We have shown that when nanocrystals are covalently bonded to glass, the linking plays a far more active part than in these earlier constructs: the glass structure can be profoundly changed, generating new amorphous structural motifs with distinct functionality.

METHODS SUMMARY
Nanocrystal-in-glass (ITO-in-NbO x ) composite films were prepared from aqueous colloidal solutions, containing polyoxometalate clusters bonded to the nanocrystals' surface. On solution deposition (spin coating) and thermal annealing (400 uC in air), polyoxometalates condense into an amorphous oxide matrix, which remains covalently linked to the embedded nanocrystals. The volume fraction of nanocrystals in the nanocomposite films was tuned by varying the polyoxometalate molar concentration in the initial colloidal solution. XRD and scanning transmission electron microscopy were used for structural characterization of the films at different stages of the thermal process. In addition, films with different interfacial densities were analysed by Raman spectroscopy to systematically follow structural changes in the amorphous matrix induced by the presence of nanocrystals. In parallel, the electrochromic properties were studied by means of spectroelectrochemical measurements. Transmittance spectra were acquired in situ, under applied voltage, by placing the film as working electrode in a liquid spectroelectrochemical cell (Li foils as counter/reference electrodes and LiClO 4 /propylene carbonate as electrolyte).
Full Methods and any associated references are available in the online version of the paper.