This paper reports the discovery of large-gap quantum spin Hall (QSH) insulators in tin films. Using first-principles calculations, the authors find that two-dimensional tin films are QSH insulators with a bulk gap of 0.3 eV, which is sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and external strain. The mechanism for the QSH effect in this system is band inversion at the Γ point, similar to the case of HgTe quantum well. With surface doping of magnetic elements, the quantum anomalous Hall effect could also be realized. Topological insulators are new states of quantum matter with insulating bulk and metallic surface/edge states protected by time-reversal symmetry. The robustness of conducting edge states in two-dimensional topological insulators has been demonstrated in the HgTe quantum well. This is promising for the realization of conducting channels without dissipation. Stoichiometric crystals such as Bi2Se3, Bi2Te3, and Sb2Te3 offer ideal model systems for the experimental investigation of three-dimensional topological insulators. However, the bulk gap in HgTe quantum well is too small, with the QSH effect observed only at low temperatures. Extensive effort has been devoted to search new QSH insulators, but desirable materials with large bulk gaps are still lacking. Following the success of graphene, various chemical classes of 2D materials have been synthesized, including novel materials initially considered to exist only in the realm of theory. The 2D group IV honeycomb lattices, interesting for electronics, have been successively fabricated. For instance, a hydrogenated graphene (graphane), a silicon counterpart of graphene (silicene), and a germanium graphane analogue have been experimentally found. Also, ultrathin tin films that are presumably to be buckled monolayer and in a honeycomb lattice were observed by early molecular beam epitaxy experiments. This Letter reports a series of large-gap QSH insulators in functionalized tin films. The chemical symbol of tin is Sn, and a monolayer of tin film can be called "stanene", in analogy with graphene and silicene. These new QSH insulators have extraordinarily large bulk gaps (~0.3 eV), their QSH states can be effectively tuned by chemical functionalization and by external strain, and their use is benefited from the abundant degree of freedom in the chemical functional group. All these make tin films intriguing for applications. The band structure of stanene is shown in Figure 2a. Two energy bands cross linearly at the K point without SOC; a band gap opens when the SOC is turned on. Thus, stanene is a QSH insulator, similar to graphene. With the stronger SOC, stanene has a larger gap of 0.1 eV. Since the graphene analogue of lead is a metalThis paper reports the discovery of large-gap quantum spin Hall (QSH) insulators in tin films. Using first-principles calculations, the authors find that two-dimensional tin films are QSH insulators with a bulk gap of 0.3 eV, which is sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and external strain. The mechanism for the QSH effect in this system is band inversion at the Γ point, similar to the case of HgTe quantum well. With surface doping of magnetic elements, the quantum anomalous Hall effect could also be realized. Topological insulators are new states of quantum matter with insulating bulk and metallic surface/edge states protected by time-reversal symmetry. The robustness of conducting edge states in two-dimensional topological insulators has been demonstrated in the HgTe quantum well. This is promising for the realization of conducting channels without dissipation. Stoichiometric crystals such as Bi2Se3, Bi2Te3, and Sb2Te3 offer ideal model systems for the experimental investigation of three-dimensional topological insulators. However, the bulk gap in HgTe quantum well is too small, with the QSH effect observed only at low temperatures. Extensive effort has been devoted to search new QSH insulators, but desirable materials with large bulk gaps are still lacking. Following the success of graphene, various chemical classes of 2D materials have been synthesized, including novel materials initially considered to exist only in the realm of theory. The 2D group IV honeycomb lattices, interesting for electronics, have been successively fabricated. For instance, a hydrogenated graphene (graphane), a silicon counterpart of graphene (silicene), and a germanium graphane analogue have been experimentally found. Also, ultrathin tin films that are presumably to be buckled monolayer and in a honeycomb lattice were observed by early molecular beam epitaxy experiments. This Letter reports a series of large-gap QSH insulators in functionalized tin films. The chemical symbol of tin is Sn, and a monolayer of tin film can be called "stanene", in analogy with graphene and silicene. These new QSH insulators have extraordinarily large bulk gaps (~0.3 eV), their QSH states can be effectively tuned by chemical functionalization and by external strain, and their use is benefited from the abundant degree of freedom in the chemical functional group. All these make tin films intriguing for applications. The band structure of stanene is shown in Figure 2a. Two energy bands cross linearly at the K point without SOC; a band gap opens when the SOC is turned on. Thus, stanene is a QSH insulator, similar to graphene. With the stronger SOC, stanene has a larger gap of 0.1 eV. Since the graphene analogue of lead is a metal