Perovskite structure and solar cell efficiencies. a Perovskites possess the general crystal structure ABX3. The most prevalent perovskite in optoelectronic devices is MA lead trihalide, for which A = CH3NH3, B = Pb, and X = Cl, Br or I. b Best research-cell efficiencies. Adapted image reproduced from NREL [6].

Synthesis of CH3NH3PbBr3 QDs. Schematic illustration of the reaction system a and precursors b. c Optical image of colloidal CH3NH3PbBr3 solution. d Schematic illustration for the formation of the CH3NH3PbBr3 QDs. Adapted image reproduced with permission of Ref. [27, 34].

Colloidal CsPbX3 perovskite NCs (X = Cl, Br, I) exhibit size- and composition-tunable bandgaps covering the entire visible spectral region with narrow and bright emission: a Colloidal solutions in toluene under UV lamp (λ = 365 nm) excitation. b Representative PL spectra. c Typical optical absorption and PL spectra. d Time-resolved PL decays for all samples shown in c except CsPbCl3. Adapted image reproduced with permission of Ref. [37].

One-step growth of perovskite NWs. a, b Slip-coating fabrication process of filiform halide perovskite and their optical image. c, f Roll-to-Roll printing for perovskite NWs thin film. d EISA method to fabricate aligned perovskite NWs. e Selective area deposition of perovskite NWs. g SEM image of NWs webs. f Transmittance spectra of NWs webs evolved with the precursor concentrations. Inset showed a photograph of semitransparent NWs networks from 30 wt% concentration. h Transmittance evolved with OTP precusor concentrations. i, j Full-inorganic CsPbBr3 NWs synthesized by one-step method and their typical optical absorption and PL spectra (j). Adapted image reproduced with permission of Ref. [49, 52, 57–59].

Two-step method for perovskite NW growth. a SEM images of vertically aligned CsPbBr3 NWs with a rectangular cross-section. b HRTEM and FFT images of a CsPbBr3 NWs. c Fluorescence decay kinetics of MAPbI3 NWs. d Conductivity improvement of the in-plane perovskite NWs. e SEM image of CH3NH3PbBr3 nanorod array. f, g Typical absorption and PL spectra of CH3NH3PbBr3 nanorod arrays. Adapted image reproduced with permission of Ref. [30, 63].

Template method for perovskite NWs: Pb-containing precursor template a method for perovskite NWs (b, c). AAO template for perovskite NWs. d Photograph of a ∼9 × 9 cm2 NW arrays. e Cross-sectional SEM images of perovskite NWs. Scale bar is 500 nm. f Average τc for different diameter NWs. g Extension of carrier lifetime as the decrease of NW diameter. h, i Nanofluidic channel template growth. Adapted image reproduced with permission of Ref. [68, 70, 71].

a Single-crystalline organometal halide perovskite nanoplatelets were synthesized by a one-pot method. Inset was the photo of the achieved colloidal perovskite nanoplatelet solution. b TEM images and selected area diffraction pattern of MAPbBr3 nanoplatelets. c X-ray diffraction of MAPbBr3 nanoplatelet film. d PL spectra of MAPbBr3 nanoplatelets in toluene solution and solid state. Inset, the optical images of MAPbBr3 nanoplatelets in toluene solution and in solid-state thin film under ambient light (left) and UV irradiation (right). e Schematic drawing of the nanoplatelet synthesis setup using a home-built vapor-transport system. Adapted image reproduced with permission of Ref. [76].

Chemical-vapor-deposited methylammonium lead halide perovskite nanoplatelets. a Schematic figure of crystal structures of CH3NH3PbI3-aXa (X = I, Br, Cl). Morphological and electronic band-edge characterizations of CH3NH3PbI3 b, CH3NH3PbI3-aBra c, and CH3NH3PbI3-aCla d nanoplatelets. Adapted image reproduced with permission of Ref. [24].

Reaction temperature influence on colloidal CsPbBr3 synthesis. 8–10 nm nanocubes were formed at 150 °C a, 20 nm nanoplates from 130 °C b reaction and 90 °C c reaction yielded several-hundred-nanometer scale lamellar nanostructures. Scale bar is 50 nm. d Absorption (solid lines) and emission (dashed lines) spectra of NPLs and nanocubes for comparison. e Colloidal solution of anion-exchanged NPLs in hexane under UV illumination (λ = 365 nm). f Inorganic perovskite PL peaks evolved with their anions. Adapted image reproduced with permission of Ref. [75]

a–c Emissions of QLED devices utilizing different QD sizes. d The EL (solid line) and the PL spectra (dashed line) of samples shown in (a–c). e CIE coordinates of the three color QLEDs (circular) compared to the NTSC color standards (star). Adapted image reproduced with permission of Ref. [90].

Device characteristics of visible PeLEDs. a Absorption (black), normalized EL (solid line in green) and PL (dashed line in green) spectra of CH3NH3PbBr3 perovskite. EL spectrum of mixed halide perovskite is shown in red. b Luminance (black) and current density (red) versus voltage characteristics of the green PeLED. c EQE versus voltage characteristics of the green PeLED. d EQE versus current density of the green PeLED. Adapted image reproduced with permission of Ref. [96].

Illustration of multilayer PeLED device. a Device structure. b Cross-sectional TEM image of multiple layers with distinct contrast. Scale bar is 50 nm. c Flat-band energy level diagram. Adapted image reproduced with permission of Ref. [90].

Perovskite NW solar cells: a, b SEM image of NWs from a one-step method and its corresponding J–V curve. Two-step-method grown NWs and corresponding device performances. c, d Top-view and cross-section of NWs films. e, f J–V and IPCE curves of NW solar cells. The scale bar is 1 m. Adapted image reproduced with permission of Ref. [52, 63].

SEM, J–V, and EQE measurements for perovskite solar cells. a Cross-section SEM image of the device. The comparison of SEM surface images of FAPbI3-based layer formed on mp-TiO2 by IEP b and conventional method c. d J–V curves of best device measured in reverse and forward modes, and e EQE spectra for best device and integrated JSC. Adapted image reproduced with permission of Ref. [101].

a One-step deposition. b Two-step sequential deposition. c Dual-source coevaporation. d Sequential vapor deposition. e Vapor-assisted solution growth process. Adapted image reproduced with permission of Ref. [18, 105–107].

Perovskite NW photodetectors: Slip-coating-based NWs PDs. a Dark and laser illuminated I–V curves. b Time-resolved photoresponse. EISA-method-based device performances. c I–t curves. d High-resolution scan to one cycle of I–t curves. Doctor-blading-based NW PDs. e Wavelength-dependent photoresponsivity of the CH3NH3PbI3 MW array-based photodetectors. f Photocurrent versus light intensity curves. g Variation of dark current/photocurrent with day of photodetector based on OTP MW arrays. h Perovskite MW PD arrays for light source mapping. NW-web-based PDs for imaging. i–j Flexibility performances. k–m NW network PD arrays for imaging. n, o Encapsulation could help to improve the device stability. Adapted image reproduced with permission of Ref. [49, 52, 60].

Perovskite NWs lasers. a–f SEM image and nanolaser performances from MAPbX3 NWs. g A set of dark field images of a IOP MW waveguide. Scale bar is 10 µm. h–i FAPbX3 NW laser performances. j–l Full-inorganic halide perovskite NW lasers. Adapted image reproduced with permission of Ref. [67, 119, 128, 129].

The mechanism for the formation of double-layer vertically aligned carbon nanotube arrays (VACNTs) through single-step CVD growth was investigated. The growth of the second CNT layer lifts the amorphous carbon coatings on catalyst particles and substrates.

Side-view SEM images of VACNTs obtained by different growth time. a 15 min (868.9 μm in height ). b 30 min (1.6 mm in height). c 30 min (1.9 mm in height and the height of the top layer is 1.5 mm) d 60 min (3.1 mm in height and the height of the top layer is 1.6 mm) e 120 min (4.6 mm in height and the height of the top layer is 1.3 mm) f TEM analysis of CNTs.

Variation of intensity ratio of G/D-band (IG/ID) of Raman spectra along the vertical direction of a double-layer VACNT array.

a SEM image showing the interface between the top layer and the bottom layer of a double-layer VACNT array. b Magnified SEM image of a. c SEM image showing the facture interface between top layer and bottom layer for double-layer VACNTs after pulling test with nippers. d Magnified SEM image of the interface.

Quantitative analysis of C, O, Al, and Fe elements on the interface between the top layer and the bottom layer as characterized by one-line scan electron microprobe technique.

Illustration showing the formation mechanism of double-layer VACNT arrays by single-step CVD method.

SEM images of the films formed by treating Ti foils at 80°C for 6 h in H2O2-NaOH(2M) mixed solutions with different H2O2 concentrations. (a) 0, (b) 7.5%v/v, (c) 11.25% v/v, and (d) 15% v/v. Inset of Fig. 2d is the corresponding TEM image of the nanowires.

(a) EDS spectra of pretreated Ti substrate, porous film formed in NaOH solution, and nanowire film formed in H2O2-NaOH mixed solution, and (b) XRD pattern of the nanowire film formed by treating Ti foils in H2O2(15%v/v)-NaOH(2M) mixed solutions for 6 h at 80°C.

SEM images of the films formed by treating Ti foils in H2O2(15%v/v)-NaOH(2M) mixed solutions for 6 h at different temperatures:(a) 40°C, (b) 60°C, (c) 100 oC, and (d) 120°C.

SEM images of the films formed by treating Ti foils at 80°C for 6 h in different solutions with the concentration of H2O2 being fixed at 15%v/v:(a) NaCl(2M)-H2O2, (b) H2O2, (c) NH3H2O(18.5%v/v)-H2O2, (d) NaCl(2M)-NH3H2O(18.5%v/v)-H2O2, (e) KOH(2M)-H2O2, and (f) Ba(OH)2 (1.5M)-H2O2.

SEM images of the films formed by treating Ti foils in H2O2 (15%v/v)-NaOH(2M) mixed solution at 80°C for different durations:(a）0, (b) 0.5 h, (c) 1 h, (d) 1.5 h, (e) 2 h, and (f) 4 h. Insets of Fig. 5b, 5c, 5d and 5e are the high magnification images of corresponding materials.

A scheme for the formation of titanate nanowires films in NaOH-H2O2 aqueous solutions.