An Experimental Research For The Optoelectronic Characteristics Of 2D Black Phosphorus

Monolayer(ML)and few-layer(FL)black phosphorus(BP)are two-dimensional(2D)semiconductors with direct band gap.They have great potential in electronics and optoelectronics due to their thickness-dependent bandgap,high carrier mobility,and anisotropic optical properties.Currently,ML-and FL-BP samples with high stability and feasibility in optoelectronic applications are mainly obtained by liquid phase exfoliation(LPE)method,such as FL-BP nanosheet and BP quantum dot(QD)dispersions.However,systematic researches for the optical absorption and emission properties of FL-BP and BP-QD dispersions are still lacking.Therefore,by tuning the concentration and centrifugal rate,we prepare FL-BP and BP-QD dispersions with different statistical distributions of BP thickness and lateral size.The absorption of visible and near-infrared(NIR)light and photoluminescence(PL)spectra of these BP dispersions,as well as the corresponding mechanisms,are investigated in detail in this thesis.The main contents and results are listed as follows:(1)FL-BP nanosheets and BP-QD dispersions are prepared by LPE with N-methyl-2-pyrrolidone(NMP)and water as solvents,respectively.Then,the morphology,structure,thickness and size distributions of the samples are characterized.It is found that by changing the centrifugal rate,the average thickness and lateral size of FL-BP nanosheets,as well as the average size of BP-QD can be adjusted.Moreover,the ratio of two characteristic Raman peaks of each sample fulfills the relation of,indicating that the degradation of 2D BP is very weak.This is because the solvation shell acts as a barrier to prevent oxidative species reaching the surfaces of FL-BP nanosheets and BP-QD.(2)By measuring the relative transmittance spectra in visible and NIR regimes(300?2000 nm),the absorbance and extinction coefficient of the 2D BP dispersions are derived.The effects of the solvent,sample concentration,and centrifugation rate on the absorption properties of the samples are discussed in detail.We find that the absorption properties of the FL-BP/BP-QD dispersions comes from the contributions of 2D BPs with different layers in the samples,which are accordingly associated with the thickness and size distributions of the BP dispertions.On one hand,the absorption peak in the visible range is strongly affected by the scattering of the BP nanosheets or QDs,which depends on the average lateral size of BPs.On the other hand,the NIR absorption peaks are mainly caused by exciton absorption,which is related to the average thickness(i.e.number of layers).(3)The PL properties of FL-BP nanosheet and BP-QD dispersions are investigated at different excitation wavelengths in a wide range(?_ex=500-700 nm).The dependence of the peak emission wavelength?_em(Peak)and peak emission intensityon the excitation wavelength?_ex are obtained,respectively.Different from previous studies focusing on a single FL-BP,the relations of?_em(Peak)-?_ex and-?_ex are determined by the average thickness and the average size of the 2D BPs in the dispersion.In addition,the PL properties of the samples are also the collective superposition effect of all the BPs in the samples,and there is competition between the light absorption and PL emission.In other words,the maximum PL of the samples can only occur when the two effects are balanced.In this thesis,we not only obtain the optical absorption parameters of the FL-BP nanosheet and BP-QD dipersions in the visible and NIR regimes,but also measure the PL properties of the samples,including?_em(Peak)-?_ex and-?_ex relations.Moreover,the effects of the solvent,concentration,and centrifugation rate on the absorption and PL properties are also discussed.These results are important supplements to the study of optoelectronics of 2D BPs in dispersions,and provide a reference for the device applications of 2D BPs.