Film Growth And Structure/Property Tuning Of Chalcogen Materials

Recently,chalcogens and chalcogenides play an increasingly significant role in condensed matter physics and material science for their plenty of physical insights and outstanding functional performance.Tellurium,the heaviest non-radioactive element in chalcogen group,introduces strong spin-orbit coupling to tellurides accompanied by a wealth of novel quantum phenomena.Furthermore,selenium is another vital non-metal element in chalcogen group,likely to combine with transition metals to form compounds with special physical properties,i.e.superconducting materials(FeSe)and charge density waves materials(NbSe2).Herein the formation of Te superstructure and the epitaxial growth of iron selenide and titanium diselenide films,as well as their structure/properties tuning have been systematic investigated by molecular beam epitaxy(MBE)and scanning tunneling microscopy/spectroscopy(STM/S).The main work and conclusions are summarized as follows,We report on a one-dimentional chain to two-dimensional(2D)high-order superstructure transition of Te adatoms on Au(111)film grown on mica with increased Te coverage.The 2D superstructure contains(?)×(?)R4.7°phase,3(?)×3(?)R10.9°phase and a honeycomb-like 3×3 superstructure.By means of in situ STM/S and ex situ angle-resolved photoemission spectroscopy,an interfacial state emerging at?-0.65 eV due to Te adsorption on Au(111)is revealed.Meanwhile,we demonstrate a strong interaction between Te adatoms and Au,tuning the electronic states of the Te-induced Au(111)surface.Our findings illustrate the coverage-driven morphological evolution and help understand the Te-Au micro-interaction,laying the foundation of fabricating more complex Te nanostructures on noble metals.We perfom MBE growth of iron selenide(FeSe)films on SrTiO3(001)substrates,and demonstrate a selective adsorption behavior of donor/acceptor(DBTTF/TCNQ)molecules at relatively low coverage via in situ low-temperature STM.More specifically,DBTTF molecules,as electron donors,tend to adsorb on the non-superconducting areas with lower electron concentration,while TCNQ molecules,as electron acceptors,tend to adsorb on the superconducting areas with higher electron concentration.Interestingly,superconductivity can be induced/suppressed by donor/acceptor(DBTTF/TCNQ)molecules on non-superconducting/superconducting FeSe layer.By means of DFT calculation,local work function and evolutions of electronic states measurement,we deduce charge transfer effect between molecules and FeSe layers,as well as a negligible structural changes induced by molecule adsorption.Our results emphasizes the role of local electron concentration in the superconductivity enhancement in FeSe films,and also pave the way towards the mechanistic understanding of superconductivity in FeSe-related materialsWe attempt to grown titanium diselenide(TiSe2)films on SrTiO3(111)substrates by MBE.Due to the strong strain effect of the substrates,TiSe2 films proceed a nearly Stranski-Krastanov(layer-plus-island)growth mode and exhibit multi-domain structure on single-layer TiSe2.Different from TiSe2 films grown on graphene/SiC reported,no signs of charge density waves(CDWs)on TiSe2 films grown on SrTiO3(111)are found by in situ STM/S,as a result of surface defects and long-range confinement caused by domain boundries and strain effect.Our work offer important insights into the mechanism of CDWs in transition metal chalcogenides,and also provide a promising strategy in MBE growth of other transition metal chalcogenides with similar atomic structure.