First Principles Study On Structures And Optical Properties Of One-dimensional ?-Si₃N₄ Nanomaterials

Silicon nitride?Si₃N₄?is an important wide-band gap semiconductor and structural material with excellent photoelectrical and thermodynamic properties,so the one-dimensional?1D?Si₃N₄ nanomaterials are considered to be good candidates for designing the electronic/optic nanodevices used in high temperature,high strength,corrosion and radiation environments.However,the preparations and performances of 1D Si₃N₄ nanomaterials have not been studied thoroughly,and still remain at the simple laboratory synthesis and characterizations,which limits their wide applications.In this paper,the effects of orientations,section sizes,surface modifications and dopings on the structural stability and electronic properties of 1D ?-Si₃N₄ nanomaterials were investigated systematically using first-principles methods,the thermodynamic equilibrium temperatures of pure 1D ?-Si₃N₄ nanomaterials were derived,and the photoelectrical properties of 1D ?-Si₃N₄ materials were analyzed.Our study lays the theoretical foundations for the wide applications of 1D Si₃N₄ nanomaterials in optoelectronic nanodevices.The 1D ?-Si₃N₄ nanostructures orientated along [001],[100],[110],[011],[101] and [210] represent the most commonly synthesized ones based on experiments.The 1 × 1 × 1,2 × 2 × 1 and 3 × 3 × 1 supercells of bulk ?-Si₃N₄ were considered,respectively.Firstly,the structural stability and electronic properties of pure 1D ?-Si₃N₄ nanostructures were studied.The results show that these 1D nanostructures could be obtained.As the section size is increased,the formation energy and total surface energy decrease,which originates from the decrease of percentage of surface dangling bonds.Meanwhile,the stability of 1D ?-Si₃N₄ nanomaterials is weakly orientation-dependent,which is in good agreement with the experimental findings that the already-synthesized 1D ?-Si₃N₄ nanomaterials could grow along various directions.In addition,according to the formation energies and thermodynamic theories,the thermodynamic formation temperatures of pure 1D ?-Si₃N₄ nanomaterials are above 1500?¹600?.The relationships among section size,orientation and thermodynamic formation temperature were established to provide the theoretical basis and guidance for the synthesis of 1D ?-Si₃N₄ nanomaterials experimentally.Due to the surface dangling bonds,the surface states exist near the Fermi level,which result in the pure 1D ?-Si₃N₄ nanomaterials exhibiting narrow band gap semiconductors or metals.The band gap increases with the decrease of section size caused by quantum confinement effect.Next,the surface dangling bonds of 1D ?-Si₃N₄ nanomaterials were saturated with H atoms,OH groups and halogen atoms?F,Cl?,respectively.It was shown that the surface modification can effectively stabilize 1D ?-Si₃N₄ nanomaterials.When the section size is close,the energetic stabilities of 1D nanomaterials depend weakly upon their orientations,but sensitively on modification treatments.The stability follows the order of OH > F > Cl > H,and changes differently with the increase of coverage for different modifications.Additionally,the surface states near the Fermi level can be removed by surface modifications,resulting in the wider band gaps.The 1D ?-Si₃N₄ nanomaterials with different surface modifications exhibit direct or indirect band characteristics,and possess different band gaps,which are attributed to a competition between quantum confinement effect and the role of surface modified groups or atoms.The former widens the band gap,while the latter narrows the gap.The Cl-modified 1D nanomaterials result in the smaller band gap values than their H-or F-modified counterparts.While the coverage of OH,F or Cl is increased to 100%,the band gaps decrease significantly,indicating an improvement of electrical properties,which is good agreement with the experimental findings.The effective masses of charge carriers of 1D ?-Si₃N₄ nanomaterials are mainly affected by the orientations and surface modifications.The [001]-orientated 1D ?-Si₃N₄ nanomaterials with 100% H modification possess the smaller effective masses and band gaps than those of ?-Si₃N₄ bulk,suggesting their good conductive properties.The [001]-orientated 1D ?-Si₃N₄ nanomaterial with 2 × 2 × 1 supercell and 100% H modification is doped with Al,Y,P and O atoms,respectively.Besides,Al+O codoping is also considered.The results show that these impurities prefer to occupy the surface of nanomaterial except Al atom,which prefers to occupy the center of nanomaterial.The Al and P atom dopings are endothermic reactions,while the O,Y atom dopings and Al+O codoping are exothermic reactions,in which the Al+O codoping is the easiest.The doping could further modulate the electronic properties of 1D ?-Si₃N₄ nanomaterials through impurity states.The Al and Y atom dopings result in the decreased band gap and p-type semiconductors,while the P and O atom dopings result in the slightly increased band gap and n-type semiconductors,and the Al+O codoped ?-Si₃N₄ nanomaterial is similar to intrinsic semiconductor with an obviously decreased band gap.All single-atom doped 1D ?-Si₃N₄ nanomaterials still exhibit direct band gaps,while the Al+O codoping causes indirect band gap.For the 1D ?-Si₃N₄ nanomaterials with different dopings,the electron and hole effective masses change in different degrees.Hole effective masses of 1D ?-Si₃N₄ nanomaterial doped with Y atom increase significantly.The optical properties of 1D ?-Si₃N₄ nanomaterials with different orientations,section sizes,surface modifications and dopings were investigated by calculating various optical parameters,including the dielectric function,absorption coefficient,reflectivity,refractivity,photoconductivity and so on.The results show that the optical properties of 1D ?-Si₃N₄ nanomaterials are anisotropic.The linear light response ranges mainly locate in the ultraviolet region of 5³0 e V,where 1D ?-Si₃N₄ nanomaterials mainly absorb and refract the light,while the reflection of light is very weak.Comparing with the [100]-and [110]-orientated nanoblets,the [001]-orientated 1D ?-Si₃N₄ nanomaterials possess the higher static optical parameters.As the section size is increased,the optical absorption edge is red shifted and the intensity of dielectric peaks increases.While the supercells are increased to 2 × 2 × 1 and 3 × 3 × 1,the number of dielectric peaks decreases,and the peak shapes and locations tend to be similar to each other.The major ?2???peaks locate at about 8.5 e V,10.5 e V and 15 e V.As the OH,F and Cl coverage is increased to 100%,the absorption edges are significantly red-shift due to the reduced band gaps,and the new dielectric peaks appear.Meanwhile,the static optical parameters,including the static dielectric constant ??0?,static reflectivity R?0?and static refractivity n?0?increase.The 100% Cl-modified nanomaterial possesses the largest static optical parameters ??0?,R?0?and n?0?,even tens of times larger than those of other nanomaterials,indicating the special applications in some materials or devices which demand the high dielectric constant,reflectivity and refractivity.The calculated photoconductivities further suggest that the 1D ?-Si₃N₄ nanomaterials have the potential applications in the field of high energy detectors.Because dopings mainly affect the band gap edges and energy levels within forbidden bands,the optical properties of doped 1D ?-Si₃N₄ nanomaterials mainly change at or below the absorption edges.Relatively speaking,for the Al and Y atoms doped 1D ?-Si₃N₄ nanomaterials,the optical properties change more obviously,and the optical parameters increase significantly near 0 e V.