Laser manufacturing of spatial resolution approaching quantum limit

Atomic and close-to-atom scale manufacturing is a promising avenue toward single-photon emitters,single-electron transistors,single-atom memory,and quantum-bit devices for future communication,computation,and sensing applications.Laser manufacturing is outstanding to this end for ease of beam manipulation,batch production,and no requirement for photomasks.It is,however,suffering from optical diffraction limits.Herein,we report a spatial resolution improved to the quantum limit by exploiting a threshold tracing and lock-in method,whereby the two-order gap between atomic point defect complexes and optical diffraction limit is surpassed,and a feature size of<5 nm is realized.The underlying physics is that the uncertainty of local atom thermal motion dominates electron excitation,rather than the power density slope of the incident laser.We show that the colour centre yield in hexagonal boron nitride is transformed from stochastic to deterministic,and the emission from individual sites becomes polychromatic to monochromatic.As a result,single colour centres in the regular array are deterministically created with a unity yield and high positional accuracy,serving as a step forward for integrated quantum technological applications.

Highly Fluorescent Semiconducting Two-Dimensional Conjugated Polymer Films Achieved by Side-Chain Engineering Showing Large Exciton Diffusion Length

Semiconducting two-dimensional conjugated polymers(2DCPs)with strong fluorescence emission have great potential for various optoelectronic applications.However,it is enormously challenging to achieve this goal due to the significant compact interlayerπ-πstacking-induced quenching effect in these systems.In this work,we found that highly fluorescent semiconducting 2DCPs can be prepared through an effective side-chain engineering approach in which interlayer spacers are introduced to reduce the fluorescence quenching effect.The obtained two truxene-based 2DCP films that,along with-C6H13 and-C_(12)H_(25)alkyl side chains as interlayer spacers both demonstrate superior fluorescence properties with a high photoluminescence quantum yield of 5.6%and 14.6%,respectively.These are among the highest values currently reported for 2DCP films.Moreover,an ultralong isotropic quasi-twodimensional exciton diffusion length constrained in the plane with its highest value approaching 110 nm was revealed by the transient photoluminescence microscopy technique,suggesting that theπ-conjugated structure in these truxene-based 2DCP films has effectively been extended.This work can enable a broad exploration of highly fluorescent semiconducting 2DCP films for more deeply fundamental properties and optoelectronic device applications.

Transient Superdiffusion of Energetic Carriers in Transition Metal Dichalcogenides Visualized by Ultrafast Pump-Probe Microscopy

Because of the strong Coulomb interaction and quantum confinement effect,2-dimensional transitionmetal dichalcogenides possess a stable excitonic population.To realize excitonic device applications,such as excitonic circuits,switches,and transistors,it is of paramount importance for understanding the optical properties of transition metal dichalcogenides.Furthermore,the strong quantum confinement in 2-dimensional space introduces exotic properties,such as enhanced phonon bottlenecking effect,many-body interaction of excitons,and ultrafast nonequilibrium exciton-exciton annihilation.Exciton diffusion is the primary energy dissipation process and a working horse in excitonic devices.In this work,we investigated time-resolved exciton propagation in monolayer semiconductors of wSe_(2),MowSe_(2),and MoSe_(2),with a home-built femtosecond pump-probe microscope.We observed ultrafast exciton expansion behavior with an equivalent diffusivity of up to 502 cm^(2)s^(-1)at the initial delay time,followed by a slow linear dffusive regime(20.9 cm^(2)s^(-1))in the monolayer WSe_(2).The fast expansion behavior is attributed to energetic carrier-dominated superdiffusive behavior.We found that in the monolayers MowSe_(2)and MoSe_(2),the energetic carrier-induced exciton expansion is much more effective,with diffusivity up to 668 and 2295 cm^(2)s^(-1),respectively.However,the"cold"exciton transport is trap limited in MowSe_(2)and MoSe_(2),leading to negative diffusion behavior at later time.Our findings are helpful to better understand the ultrafast nonlinear diffusive behavior in strongly quantum-confined systems.It may be harnessed to break the limit of conventional slow diffusion of excitons for advancing more efficient and ultrafast optoelectronicdevices.

Photoexcitation dynamics in solution-processed formamidinium lead iodide perovskite thin films for solar cell applications

Formamidinium lead iodide(FAPbI3)is a newly developed hybrid perovskite that potentially can be used in high-efficiency solution-processed solar cells.Here,the temperature-dependent dynamic optical properties of three types of FAPbI3 perovskite films(fabricated using three different precursor systems)are comparatively studied.The time-resolved photoluminescence(PL)spectra reveal that FAPbI3 films made from the new precursor(a mixture of formamidinium iodide and hydrogen lead triiodide)exhibit the longest lifetime of 439 ns at room temperature,suggesting a lower number of defects and lower non-radiative recombination losses compared with FAPbI3 obtained from the other two precursors.From the temperature-dependent PL spectra,a phase transition in the films is clearly observed.Meanwhile,exciton-binding energies of 8.1 and 18 meV for the high-and lowtemperature phases are extracted,respectively.Importantly,the PL spectra for all of the samples show a single peak at room temperature,whereas at liquid-helium temperature the emission features two peaks:one in higher energy displaying a fast decay(0.5 ns)and a second red-shifted peak with a decay of up to several microseconds.These two emissions,separated by~18 meV,are attributed to free excitons and bound excitons with singlet and triplet characters,respectively.