Surface treatment of GaN nanowires for enhanced photoelectrochemical water-splitting

High-efficiency hydrogen production through photoelectrochemical(PEC)water splitting has emerged as a promising solution to address current global energy challenges.Ⅲ-nitride semiconductor photoelectrodes with nanostructures have demonstrated great potential in the near future due to their high light absorption,tunable direct band gap,and strong physicochemical stability.However,several issues,including surface trapping centers,surface Fermi level pinning,and surface band bending,need to be addressed.In this work,enhanced photovoltaic properties have been achieved using gallium nitride(GaN)nanowires(NWs)photoelectrodes by adopting an alkaline solution surface treatment method to reduce the surface states.It was found that surface oxides on NWs can be removed by an alkaline solution treatment without changing the surface morphology through X-ray photoelectron spectroscopy(XPS),scanning electron microscopy(SEM)and other characterization methods.These findings provide new insights to the development of high-efficiency photoelectrodes for new energy source applications.

Three distinct optical-switching states in phase-change materials containing impurities:From physical origin to material design

Ge2 Sb2 Te5 is the most widely utilized chalcogenide phase-change material for non-volatile photonic applications,which undergoes amorphous-cubic and cubic-hexagonal phase transition under external excitations.However,the cubic-hexagonal optical contrast is negligible,only the amorphous-cubic phase transition of Ge_(2)Sb_(2)Te_(5) is available.This limits the optical switching states of traditional active displays and absorbers to two.We find that increasing structural disorder difference of cubic-hexagonal can increase optical contrast close to the level of amorphous-cubic.Therefore,an amorphous-cubichexagonal phase transition with high optical contrast is realized.Using this phase transition,we have developed display and absorber with three distinct switching states,improving the switching performance by 50%.Through the combination of first-principle calculations and experiments,we reveal that the key to increasing structural disorder difference of amorphous,cubic and hexagonal phases is to introduce small interstitial impurities(like N)in Ge2 Sb2 Te5,rather than large substitutional impurities(like Ag)previously thought.This is explained by the formation energy and lattice distortion.Based on the impurity atomic radius,interstitial site radius and formation energy,C and B are also potential suitable impurities.In addition,introducing interstitial impurities into phase-change materials with van der Waals gaps in stable phase such as GeSb_(4) Te_(7),GeSb_(2) Te_(4),Ge_(3)Sb_(2) Te_(6),Sb_(2)Te_(3) will produce high optical contrast amorphous-metastable-stable phase transition.This research not only reveals the important role of interstitial impurities in increasing the optical contrast between metastable-stable phases,but also proposes varieties of candidate matrices and impurities.This provides new phase-change materials and design methods for non-volatile optical devices with multi-switching states.