Investigations Of The Opto-electronical Properties Of Twod-imensional Gallium Selenide And Metal Chalcogenides

Gallium selenide(GaSe)is a typical layered A^IIIB^VI binary chalcogenides.Different from the semimetallic graphene(zero band gap)and insulating hexagonal boron,Ga Se is especially interesting for its nonzero band gap(semiconductor)and strong photoluminescence(PL)in single-layer form,which demonstrates promising applications in piezo-phototropics,optoelectronics,wearable devices and human-machine interfaces.In this work,we systematically investigate the Ga Se from the aspects of fabrication,characterization,electronics,photonics,thermotics,optoelectronics and piezo-phototropics.(1)We investigated the effect of van der Waals stacking on the lattice vibrations of Ga Se,paying particular attention to its temperature dependence.We show that the out of plane vibrations become less confined when the number of layers within the stack is decreased.With decreasing layer numbers,the out of plane vibrational modes show a red-shift,and the intensity ratio of the A'₁(1¹)and A'₁(2²)peaks decreases.The shift of major peaks and the intensity ratio of the A'₁(1¹)and A'₁(2²)modes can be used as convenient indications to rapidly characterize the thickness of Ga Se.Notably,a negative correlation between the magnitude of the out-of-plane first-order temperature coefficient and thickness was observed,with the coefficient for monolayer Ga Se being almost double that of bulk Ga Se.Furthermore,we demonstrated the viability of Ga Se based gas sensors.An experimentally determined minimum detection limit of 4 ppm for NH₃was found,which demonstrates sensitivity comparable with many other 2D materials.(2)We investigated the strain-induced tunable optical properties of Ga Se.At room temperature,a linear tuning rate for the optical band gap of 40 me V/1%is achieved for elastic strain.Importantly,excitons move from both the flat regions and inner circumference of the Ga Se wrinkles and concentrated at the surface of the Ga Se wrinkles,resulting in a 3-fold enhancement of the PL intensity for a small strain of?1%.We expect that strain engineering of Ga Se,which can be simply generated by bending the nano-sheets,could provide a new platform for its application in tunable energy harvesting,and conversion,and flexible opto-electromechanical applications.(3)The temperature-dependent PL from the strained and unstrained Ga Se flakes have been systematically investigated.As the temperature increase,the PL from both the strained(wrinkle)and unstrained(flat)positions show a prominent red-shift to low energies.But for the flat case,the slope of PL energy versus temperature at the range of 163-283 K is about0.36 me V/K,which is smaller than that of wrinkle one(0.5 me V/K).This can be understood by the fact that more strain can be introduced at the freestanding wrinkle position during the temperature increasing,thus accelerates the major PL peak shift to lower energy.Additionally,for the wrinkle sheet,three new exciton states(peaks III,IV and V)appear at the red side of the main PL peak I,and the emission intensity is highly dependent on the temperature variation.This peak can be attributed to the bound excitons recombination.These findings opens a new interesting route for optical band gap tuning of the layered Ga Se sheet,which is important for the future optoelectronic devices designing.(4)We investigate the optoelectronic properties of Ga Se nanosheets under elastic strain.By studying the piezoelectricity of a Ga Se nanosheet through its opto-electrical properties a large piezo-resistive effect was found,which has the effect of enhancing the optoelectronic performance of the Ga Se nanosheets.Indeed,it was found that the PL intensity is enhanced linearly with the applied strain and a 3-fold enhancement was achieved under a strain of 0.3%.Furthermore,a large increase of conductivity and photocurrent is observed,which increases linearly with the applied strain.Remarkably,an enhancement factor of more 240 for the current was reliably was achieved by manually bending Ga Se based devices.This large enhancement is attributed to the local exciton potential induced by the elastic strain gradient which drives the electrons(holes)to move towards the outer(inner)surface of the wrinkles.This work suggests that Ga Se is a potential material for flexible nano-devices and flexible nano-optoelectronics.(5)We investigated the temperature dependent surface-enhanced Raman scattering(SERS)of monolayer MX₂.We found that the SERS depends crucially on the distribution of the metallic NPs and also the phonon mode of the MX₂.Strong coupling between MX₂ and metallic NPs,through surface plasmon excitation,results in splitting of theE'((38))and₁A'((38))mode and additional peaks become apparent.For a WS2-Ag system,the intensity of the additional peaks increases linearly with local strain.Moreover,due to the monolayer's nature,the thermal properties,such as thermal stability and thermal conductivity,of the MX₂ can be tuned by metallic NPs on top.When 5 nm thick Ag NPs were deposited on top,the first-order temperature coefficient of Raman shift of WS₂ is doubled.Our experimental study is a step towards the application of monolayer MX₂ in flexible nano-optoelectronics.