Tellurium (Te) films with monolayer and few-layer thickness are obtained by molecular beam epitaxy on a graphene/6H-SiC(0001) substrate and investigated by in situ scanning tunneling microscopy and spectroscopy (STM/STS). We reveal that the Te films are composed of parallel-arranged helical Te chains flat-lying on the graphene surface, exposing the (1 × 1) facet of (101̅0) of the bulk crystal. The band gap of Te films increases monotonically with decreasing thickness, reaching the near-infrared band for the monolayer Te. An explicit band bending at the edge between the monolayer Te and graphene substrate is visualized. With the thickness controlled in the atomic scale, Te films show potential applications of electronics and optoelectronics.

No abstract

Long persistent luminescence (LPL) materials have a wide range of applications, such as in architectural decorations, safety signs, watch dials, and glow-in-the-dark toys. Present LPL materials based on inorganics must be processed into powders and blended with polymer matrices before use. However, micro powders of inorganic LPL materials show poor compatibility with common polymers, limiting the mechanical properties and transparency of the composites. Here, we report a polymer-based organic LPL (OLPL) system that is flexible, transparent, and solution-processable.Following low-power excitation at room temperature, this polymer-based OLPL system exhibits LPL after phosphorescence from the donor.Long persistent luminescence (LPL), also called long afterglow, is the phenomenon when, after being excited, a material in the dark emits light for several seconds, minutes, hours, or even days. [1] Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

Organic long-persistent-luminescent (OLPL) materials demonstrating hour-long photoluminescence have practical advantages in applications owing to their flexible design and easy processability. However, the energy absorbed in these materials is typically stored in an intermediate charge-separated state that is unstable when exposed to oxygen, thus preventing persistent luminescence in air unless oxygen penetration is suppressed through crystallization. Moreover, OLPL materials usually require ultraviolet excitation. Here we overcome such limitations and demonstrate amorphous OLPL systems that can be excited by radiation up to 600 nm and exhibit persistent luminescence in air. By adding cationic photoredox catalysts as electron-accepting dopants in a neutral electron-donor host, stable charge-separated states are generated by hole diffusion in these blends. Furthermore, the addition of hole-trapping molecules extends the photoluminescence lifetime. By using a p-type host less reactive to oxygen and tuning 2 the donor-acceptor energy gap, our amorphous blends exhibit persistent luminescence stimulated by visible light even in air, expanding the applicability of OLPL materials. MainLong-persistent luminescence (LPL) is a phenomenon in which emission continues for a long period after photoexcitation 1 . LPL emitters are used as glow-in-the-dark paints for clock faces and emergency lights. High-efficiency LPL materials are composed of metal-oxide microcrystals and small amounts of rare-earth ions that act as chargetrapping and emission sites 2,3 . In these inorganic LPL materials, holes or electrons generated by the photoexcitation of the metal-oxide crystal are accumulated in dopants that act as charge-trapping sites. Gradual charge recombination followed by thermal detrapping produces hour-long emissions 4,5,6 . Inorganic LPL materials are insoluble in any solvent, and require grinding into microparticles before dispersion into solvents or polymers for practical applications. Most inorganic LPL systems require ultraviolet to blue excitation light below 450 nm due to the limited metal-oxide absorption bands 7,8,9 .We have reported LPL emissions from mixtures of organic molecules 10 . This organic LPL (OLPL) system can be fabricated from a solution process 11 and the fabricated films can be transparent and flexible 12 . LPL emissions can be tuned from greenish-blue to red with the addition of fluorescent materials 13 . In contrast to conventional organic roomtemperature phosphorescent (RTP) materials 14 , which store their energy in triplet excited states and exhibit radiative transition from the triplet excited states to the singlet ground states 15 , OLPL systems accumulate energy into charge-separated states similar to inorganic LPL materials. LPL and RTP can be identified from their emission decay profiles 16,17,18 .Current OLPL systems require inert gas conditions to exhibit LPL because LPL is