On-site growth of perovskite nanocrystal arrays for integrated nanodevices.

Despite remarkable progress in the development of halide perovskite materials and devices, their integration into nanoscale optoelectronics has been hindered by a lack of control over nanoscale patterning. Owing to their tendency to degrade rapidly, perovskites suffer from chemical incompatibility with conventional lithographic processes. Here, we present an alternative, bottom-up approach for precise and scalable formation of perovskite nanocrystal arrays with deterministic control over size, number, and position. In our approach, localized growth and positioning is guided using topographical templates of controlled surface wettability through which nanoscale forces are engineered to achieve sub-lithographic resolutions. With this technique, we demonstrate deterministic arrays of CsPbBr3 nanocrystals with tunable dimensions down to <50 nm and positional accuracy <50 nm. Versatile, scalable, and compatible with device integration processes, we then use our technique to demonstrate arrays of nanoscale light-emitting diodes, highlighting the new opportunities that this platform offers for perovskites' integration into on-chip nanodevices.

Vertical full-colour micro-LEDs via 2D materials-based layer transfer.

Micro-LEDs (µLEDs) have been explored for augmented and virtual reality display applications that require extremely high pixels per inch and luminance1,2. However, conventional manufacturing processes based on the lateral assembly of red, green and blue (RGB) µLEDs have limitations in enhancing pixel density3-6. Recent demonstrations of vertical µLED displays have attempted to address this issue by stacking freestanding RGB LED membranes and fabricating top-down7-14, but minimization of the lateral dimensions of stacked µLEDs has been difficult. Here we report full-colour, vertically stacked µLEDs that achieve, to our knowledge, the highest array density (5,100 pixels per inch) and the smallest size (4 µm) reported to date. This is enabled by a two-dimensional materials-based layer transfer technique15-18 that allows the growth of RGB LEDs of near-submicron thickness on two-dimensional material-coated substrates via remote or van der Waals epitaxy, mechanical release and stacking of LEDs, followed by top-down fabrication. The smallest-ever stack height of around 9 µm is the key enabler for record high µLED array density. We also demonstrate vertical integration of blue µLEDs with silicon membrane transistors for active matrix operation. These results establish routes to creating full-colour µLED displays for augmented and virtual reality, while also offering a generalizable platform for broader classes of three-dimensional integrated devices.

Printed Organic Photovoltaic Modules on Transferable Ultra-thin Substrates as Additive Power Sources.

Thin-film photovoltaics with functional components on the order of a few microns, present an avenue toward realizing additive power onto any surface of interest without excessive addition in weight and topography. To date, demonstrations of such ultra-thin photovoltaics have been limited to small-scale devices, often prepared on glass carrier substrates with only a few layers solution-processed. We demonstrate large-area, ultra-thin organic photovoltaic (PV) modules produced with scalable solution-based printing processes for all layers. We further demonstrate their transfer onto light-weight and high-strength composite fabrics, resulting in durable fabric-PV systems ∼50 microns thin, weighing under 1 gram over the module area (corresponding to an area density of 105 g m-2 ), and having a specific power of 370 W kg-1 . Integration of the ultra-thin modules onto composite fabrics lends mechanical resilience to allow these fabric-PV systems to maintain their performance even after 500 roll-up cycles. This approach to decouple the manufacturing and integration of photovoltaics enables new opportunities in ubiquitous energy generation.

A versatile acoustically active surface based on piezoelectric microstructures.

We demonstrate a versatile acoustically active surface consisting of an ensemble of piezoelectric microstructures that are capable of radiating and sensing acoustic waves. A freestanding microstructure array embossed in a single step on a flexible piezoelectric sheet of polyvinylidene fluoride (PVDF) leads to high-quality acoustic performance, which can be tuned by the design of the embossed microstructures. The high sensitivity and large bandwidth for sound generation demonstrated by this acoustically active surface outperform previously reported thin-film loudspeakers using PVDF, PVDF copolymers, or voided charged polymers without microstructures. We further explore the directivity of this device and its use on a curved surface. In addition, high-fidelity sound perception is demonstrated by the surface, enabling its microphonic application for voice recording and speaker recognition. The versatility, high-quality acoustic performance, minimal form factor, and scalability of future production of this acoustically active surface can lead to broad industrial and commercial adoption for this technology.

Hybrid Approach to Fabricate Uniform and Active Molecular Junctions.

Molecules can serve as ultimate building blocks for extreme nanoscale devices. This requires their precise integration into functional heterojunctions, most commonly in the form of metal-molecule-metal architectures. Structural damage and nonuniformities caused by current fabrication techniques, however, limit their effective incorporation. Here, we present a hybrid fabrication approach enabling uniform and active molecular junctions. A template-stripping technique is developed to form electrodes with sub-nanometer smooth surfaces. Combined with dielectrophoretic trapping of colloidal nanorods, uniform sub-5 nm junctions are achieved. Uniquely, in our design, the top contact is mechanically free to move under an applied stimulus. Using this, we investigate the electromechanical tuning of the junction and its tunneling conduction. Here, the molecules help control sub-nanometer mechanical modulation, which is conventionally challenging due to instabilities caused by surface adhesive forces. Our versatile approach provides a platform to develop and study active molecular junctions for emerging applications in electronics, plasmonics, and electromechanical devices.

Continuous-wave lasing in colloidal quantum dot solids enabled by facet-selective epitaxy.

Colloidal quantum dots (CQDs) feature a low degeneracy of electronic states at the band edges compared with the corresponding bulk material, as well as a narrow emission linewidth. Unfortunately for potential laser applications, this degeneracy is incompletely lifted in the valence band, spreading the hole population among several states at room temperature. This leads to increased optical gain thresholds, demanding high photoexcitation levels to achieve population inversion (more electrons in excited states than in ground states-the condition for optical gain). This, in turn, increases Auger recombination losses, limiting the gain lifetime to sub-nanoseconds and preventing steady laser action. State degeneracy also broadens the photoluminescence linewidth at the single-particle level. Here we demonstrate a way to decrease the band-edge degeneracy and single-dot photoluminescence linewidth in CQDs by means of uniform biaxial strain. We have developed a synthetic strategy that we term facet-selective epitaxy: we first switch off, and then switch on, shell growth on the (0001) facet of wurtzite CdSe cores, producing asymmetric compressive shells that create built-in biaxial strain, while still maintaining excellent surface passivation (preventing defect formation, which otherwise would cause non-radiative recombination losses). Our synthesis spreads the excitonic fine structure uniformly and sufficiently broadly that it prevents valence-band-edge states from being thermally depopulated. We thereby reduce the optical gain threshold and demonstrate continuous-wave lasing from CQD solids, expanding the library of solution-processed materials that may be capable of continuous-wave lasing. The individual CQDs exhibit an ultra-narrow single-dot linewidth, and we successfully propagate this into the ensemble of CQDs.

Colloidal CdSe(1-x)S(x) Nanoplatelets with Narrow and Continuously-Tunable Electroluminescence.

Colloidal nanoplatelets, quasi-two-dimensional quantum wells, have recently been introduced as colloidal semiconductor materials with the narrowest known photoluminescence line width (∼10 nm). Unfortunately, these materials have not been shown to have continuously tunable emission but rather emit at discrete wavelengths that depend strictly on atomic-layer thickness. Herein, we report a new synthesis approach that overcomes this issue: by alloying CdSe colloidal nanoplatelets with CdS, we finely tune the emission spectrum while still leveraging atomic-scale thickness control. We proceed to demonstrate light-emitting diodes with sub-bandgap turn-on voltages (2.1 V for a device emitting at 2.4 eV) and the narrowest electroluminescence spectrum (FWHM ∼12.5 nm) reported for colloidal semiconductor LEDs.