Browsing by Author "Aktürk, E."
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Item Open Access Above room temperature ferromagnetism in Gd2B2 monolayer with high magnetic anisotropy(American Chemical Society, 2020) Görkan, T.; Vatansever, E.; Akıncı, Ü.; Gökoğlu, G.; Aktürk, E.; Çıracı, SalimThe realization of 2D ultrathin crystals with a ferromagnetic ground state that is sustainable at room temperature has been a real challenge now. By combining ab initio density functional theory with Monte Carlo simulations, we predicted a new 2D structure, Gd2B2 monolayer, which maintains its mechanical stability at elevated temperatures. More remarkably, it has a ferromagnetic ground state with high permanent magnetic moment, which persists far above room temperature. It exhibits high magnetocrystalline anisotropy along particular directions. We find also that both its magnetic anisotropy and Curie temperature can largely be altered by applied strain providing an excellent magnetoelastic tunability. This novel 2D crystal with high magnetic moment and Curie temperature combined with high structural and thermal stability can offer critical applications in magnetoelectronics.Item Open Access Adsorption of carbon adatoms to graphene and its nanoribbons(AIP Publishing, 2011) Ataca, C.; Aktürk, E.; Şahin, H.; Çıracı, SalimThis paper investigates the adsorption of carbon adatoms on graphene and its nanoribbons using first-principles plane wave calculations within density functional theory. The stability at high carbon adatom coverage, migration, and cluster formation of carbon atoms are analyzed. Carbon adatoms give rise to important changes in electronic and magnetic properties even at low coverage. While bare graphene is nonmagnetic semimetal, it is metallized and acquires magnetic moment upon coverage of carbon adatoms. Calculated magnetic moments vary depending on the coverage of adatoms even for large adatom-adatom distances. Electronic and magnetic properties of hydrogen passivated armchair and zigzag nanoribbons show strong dependence on the adsorption site. We also predict a new type of carbon impurity defect in graphene, which has a small formation energy. Interactions between distant carbon adatoms imply a long ranged interaction.Item Open Access Chemical and substitutional doping, and anti-site and vacancy formation in monolayer AlN and GaN(Royal Society of Chemistry, 2018) Kadıoğlu, Yelda; Ersan, Fatih; Kecik, Deniz; Aktürk, O. Ü.; Aktürk, E.; Çıracı, SalimWe investigated the effects of chemical/substitutional doping, hydrogenation, and anti-site and vacancy defects on the atomic, optoelectronic and magnetic properties of AlN and GaN monolayers. Upon doping of selected atoms, AlN and GaN monolayers can acquire magnetic properties, and their fundamental band gaps are modified by the localized gap states. Spin-polarized gap states broaden into bands at patterned coverage of adatoms, whereby half-metallic or magnetic semiconducting properties can be attained. Specific adatoms adsorbed to Ga atoms break the nearest vertical Ga-N bonds in the GaN bilayer in the heackelite structure and result in changes in the electronic and atomic structure. While adjacent and distant pairs of anion + cation vacancies induce spin polarization with filled and empty gap states, anti-site defects remain nonmagnetic; but both defects induce dramatic changes in the band gap. Fully hydrogenated monolayers are stable only for specific buckled geometries, where one geometry can also lead to an indirect to direct band gap transition. Also, optical activity shifts to the ultra-violet region upon hydrogenation of the monolayers. While H2 and O2 molecules are readily physisorbed on the surfaces of the monolayers with weak van der Waals attraction, they can be dissociated into constituent atoms at the vacancy site of the cation. Our study performed within density functional theory shows that the electronic, magnetic and optical properties of AlN and GaN monolayers can be tuned by doping and point defect formation in order to acquire diverse functionalities.Item Open Access Columnar antiferromagnetic order of a MBene monolayer(American Physical Society, 2021-04-16) Ozdemir, I.; Kadioglu, Y.; Yüksel, Y.; Akıncı, Ü.; Üzengi Aktürk, O.; Aktürk, E.; Çıracı, SalimFirst-principles density functional theory, combined with the Monte Carlo method, predicts that the Fe2B2 monolayer of the MBene family has a stable columnar antiferromagnetic (AFM) ground state. Below the critical temperature, Tc=115 K in equilibrium, the spins rotate by the same amount in every other column of Fe atoms, but they retain the same direction in the same column. Under applied tensile strains, Tc and the order parameter can increase nonmonotonically. The onset of the columnar order can result in a transition from two dimension (2D) to 1D in magnetic, electronic, and conduction properties. The ordered magnetic state itself can be tuned by external magnetic field, whereby the columnar magnetic order changes to ferromagnetic order with a double hysteresis behavior. When terminated by Fluorine atoms, the columnar order changes to the AFM order with Tc rising above room temperature. This situation is rather unusual and insofar is fundamental for a realistic, strictly 2D monolayer and can have critical consequences in spin conduction.Item Open Access A comparative study of lattice dynamics of three-and two-dimensional MoS2(American Chemical Society, 2011) Ataca, C.; Topsakal, M.; Aktürk, E.; Çıracı, SalimThis paper presents a comparative study of the lattice dynamics of three-dimensional layered MoS2 and two-dimensional single layer MoS2 based on the density functional theory. A comprehensive analysis of energetics and optimized structure parameters is performed using different methods. It is found that the van der Waals attraction between layers of three-dimensional (3D) layered MoS2 is weak but is essential to hold the layers together with the equilibrium interlayer spacing. Cohesive energy, phonon dispersion curves, and corresponding density of states and related properties, such as Born-effective charges, dielectric constants, Raman and infrared active modes are calculated for 3D layered as well as 2D single layer MoS2 using their optimized structures. These calculated values are compared with the experimental data to reveal interesting dimensionality effects. The absence of a weak interlayer interaction in 2D single layer MoS2 results in the softening of some of Raman active modes. © 2011 American Chemical Society.Item Open Access Deformed octagon-hexagon-square structure of group-IV and group-V elements and III-V compounds(American Physical Society, 2019) Görkan, T.; Aktürk, E.; Çıracı, SalimWe report the prediction of a two-dimensional (2D) allotrope common to group-IV and group-V elements and III-V compounds, which consist of two nonplanar atomic layers connected by vertical bonds and form deformed octagon, hexagon, and squares (dohs) with threefold and fourfold coordinated atoms. Specifically for silicon, it is a semiconductor with cohesion stronger than silicene and can be chemically doped to have localized donor and acceptor states in the band gap. This allotrope can be functionalized to construct quasi-2D clathrates with transition metal atoms and attain spin polarized metallic, half-metallic, or semiconducting states. It is demonstrated that these properties can be maintained, when it is grown on a specific substrate. Stringent tests show that the atomic structure is dynamically stable and can sustain thermal excitation at high temperatures. Additionally, stable bilayer, as well as 3D layeredlike structures, can be constructed by the vertical stacking of single-layer dohs. Surprisingly, C, Ge, AlP, and GaAs can form also similar 2D semiconducting structures. In contrast to semiconducting black and blue phosphorene, P-dohs is a semimetal with band inversion. While the premise of using well-developed silicon technology in 2D electronics has been hampered by the semimetallic silicene, the realization of this 2D, semiconducting allotrope of silicon and compounds can constitute a productive direction in 2D nanoelectronics/spintronics.Item Open Access Dipeptide adsorption on Si (100)-2x1 asymmetric surface by first principles(World Scientific Publishing Co. Pte. Ltd, 2010) Aktürk, E.; Gülseren, O.; Arkin, H.; Çelik, T.The adsorption of alanine dipeptide on a Si(100)-2 × 1 asymmetric surface is studied by using pseudopotential plane wave approach based on Density Functional Theory (DFT). Adsorption energies for different surface sites of various conformations are calculated and the groove site is found to be energetically most favorable. We observed that the molecule-surface interactions might modify surface reconstruction: asymmetric surface dimers reconstruct to asymmetric dimers in opposite directions doubling the surface periodicity, which in turn gives the surface metallic properties.Item Open Access Effects of adatoms and physisorbed molecules on the physical properties of antimonene(American Physical Society, 2016) Üzengi, Aktürk O.; Aktürk, E.; Çıracı, SalimA recent study predicted that a 2D single layer of antimony in buckled honeycomb as well as asymmetric washboard structures, named antimonene, is stable at high temperature and displays semiconducting properties. Based on first-principles, spin-polarized density functional calculations, we investigated chemisorption of selected adatoms and physisorption of molecules on two antimonene phases. Since adspecies-adspecies interaction is minimized by using large supercells, our results mimic the effects of isolated, single adatoms or molecules. We found that molecules such as H2,O2, and H2O neither form strong chemical bonds nor dissociate; they are physisorbed with a weak binding energy without affecting the properties of antimonene. The adatoms, such as H, Li, B, C, N, O, Al, In, Si, P, Cl, Ti, As, and Sb, are chemisorbed with significant binding energy, whereby the atomic and electronic structures are modified locally. Boron and carbon adatoms are implemented into buckled antimonene crystal leading to a local reconstruction of the crystal. Nitrogen gives rise to Stone-Wales type defects. The localized states originating from adatoms give rise to diversity of electronic structure. The lowest conduction and highest valence bands of antimonene in asymmetric washboard structures have very high curvature. Once combined with adatom states, these bands offer a variety of features. Specific adatoms lead to spin polarization, attain magnetic moments, and can attribute a half-metallic character to antimonene.Item Open Access Effects of silicon and germanium adsorbed on graphene(A I P Publishing LLC, 2010) Aktürk, E.; Ataca, C.; Çıracı, SalimBased on the first-principles plane wave calculations, we studied the adsorption of Si and Geon graphene. We found that these atoms are bound to graphene at the bridge site with a significant binding energy, while many other atoms are bound at the hollow site above the center of hexagon. It is remarkable that these adatoms may induce important changes in the electronic structure of graphene even at low coverage. Semimetallic graphene becomes metallized and attains a magnetic moment. The combination of adatom orbitals with those of ππ- and π∗π∗-states of bare graphene is found responsible for these effects.Item Open Access Exploring the electronic and magnetic properties of new metal halides from bulk to two-dimensional monolayer: RuX3 (X = Br, I)(Elsevier, 2018) Ersan, Fatih; Vatansever, E.; Sarikurt, S.; Yüksel, Y.; Kadıoğlu, Yelda; Ozaydin, H.; Aktürk, O. Ü.; Akıncı, Ü.; Aktürk, E.Theoretical and experimental studies present that metal halogens in MX3 forms can show very interesting electronic and magnetic properties in their bulk and monolayer phases. Many MX3 materials have layered structures in their bulk phases, while RuBr3 and RuI3 have one-dimensional chains in plane. In this paper, we show that these metal halogens can also form two-dimensional layered structures in the bulk phase similar to other metal halogens, and cleavage energy values confirm that the monolayers of RuX3 can be possible to be synthesized. We also find that monolayers of RuX3 prefer ferromagnetic spin orientation in the plane for Ru atoms. Their ferromagnetic ground state, however, changes to antiferromagnetic zigzag state after U is included. Calculations using PBE + U with SOC predict indirect band gap of 0.70 eV and 0.32 eV for the optimized structure of RuBr3 and RuI3, respectively. Calculation based on the Monte Carlo simulations reveal interesting magnetic properties of RuBr3, such as large Curie temperature against RuI3, both in bulk and monolayer cases. Moreover, as a result of varying exchange couplings between neighboring magnetic moments, magnetic properties of RuBr3 and RuI3 can undergo drastic changes from bulk to monolayer. We hope our findings can be useful to attempt to fabricate the bulk and monolayer of RuBr3 and RuI3.Item Open Access First-principles approach to monitoring the band gap and magnetic state of a graphene nanoribbon via its vacancies(American Physical Society, 2008) Topsakal, M.; Aktürk, E.; Sevinçli, H.; Çıracı, SalimUsing first-principles plane-wave calculations we predict that electronic and magnetic properties of graphene nanoribbons can be modified by the defect-induced itinerant states. Structure optimization gives rise to significant reconstruction of atomic structure, which is in good agreement with transmission electron microscope images. The band gaps of armchair nanoribbons can be modified by hydrogen-saturated holes. The band-gap changes depend on the width of the ribbon as well as on the position of the hole relative to the edges of the ribbon. Defects due to periodically repeating vacancy or divacancies induce metallization as well as magnetization in nonmagnetic semiconducting nanoribbons due to the spin polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to the edges. It is shown that these effects can, in fact, be realized without really creating defects.Item Open Access First-principles study of the iron pnictide superconductor BaFe2 As2(American Physical Society, 2009) Aktürk, E.; Çıracı, SalimThis paper presents our study on the atomic, electronic, magnetic structures, and phonon modes of the low-temperature orthorhombic phase of undoped BaFe2As2 crystal. The electronic structure is characterized by a sharp Fe-3d peak close to the Fermi level and is dominated by Fe-3d- and As-4p-hybridized states. Ba contribution occurs only at lower energies. The spin ordering of the magnetic ground state, which is determined by minimizing the total energy of different spin alignments on Fe atoms in the conventional cell, is in agreement with experimental findings but is different from the antiferromagnetic spin ordering obtained by assigning antiparallel spin directions on two Fe atoms in the primitive unit cell. Valuable information about the charge transfer and bonding is revealed through the analysis of the charge density. Electrons are transferred from Ba to Fe-As layers and also from Fe to As atoms. The magnetic phonon calculations of the ground state are carried out to predict Raman and infrared-active modes. Softening of some calculated spin-dependent phonon modes corroborates the contribution of spin-lattice coupling to the structural phase transition from I4/mmm to Fmmm.Item Open Access First-principles study of two-and one-dimensional honeycomb structures of boron nitride(American Physical Society, 2009) Topsakal, M.; Aktürk, E.; Çıracı, SalimThis paper presents a systematic study of two- and one-dimensional honeycomb structures of boron nitride BN using first-principles plane-wave method. In order to reveal dimensionality effects, a brief study of all allotropic forms of three-dimensional 3D BN crystals and truly one-dimensional atomic BN chains are also included. Two-dimensional 2D graphenelike BN is a wide band-gap semiconductor with ionic bonding through significant charge transfer from B to N. Phonon-dispersion curves demonstrate the stability of 2D BN flakes. Quasi-one-dimensional 1D armchair BN nanoribbons are nonmagnetic semiconductors with edge states. Upon passivation of B and N with hydrogen atoms these edge states disappear and the band gap increases. Bare zigzag BN nanoribbons are metallic but become a ferromagnetic semiconductor when both their edges are passivated with hydrogen. However, their magnetic ground state, electronic band structure, and band gap are found to be strongly dependent on whether B or N edge of the ribbon is saturated with hydrogen. Vacancy defects in armchair and zigzag nanoribbons affect also the magnetic state and electronic structure. Harmonic, anharmonic, and plastic regions are deduced in the variation in the total energy of armchair and zigzag nanoribbons as a function of strain. The calculated force constants display a Hookian behavior. In the plastic region the nanoribbon is stretched, whereby the honeycomb structure of hexagons change into different polygons through sequential structural transformations. In order to reveal dimensionality effects these properties are contrasted with those of various 3D BN crystals and 1D BN atomic chain.Item Open Access Free-standing and supported phosphorene nanoflakes: Shape- and size-dependent properties(Elsevier, 2020-01-01) Özaydın, H. D.; Görkan, T.; Bakir, M. Y.; Üzengi Aktürk, O.; Gökoğlu, G.; Aktürk, E.; Çıracı, SalimThe ultra-small sized nanomaterials are important for basic functional components of future nanoelectronics, spintronics and sensor devices. In this study, based on first-principles density functional theory, the free-standing and supported nanoflakes of bare and hydrogen saturated black and blue phosphorene of diverse size and shape have been investigated. Cohesion, formation energy, thermal stability and electronic structure of these nanoflakes have been revealed. For nanoflakes supported by specific substrates, such as phosphorene, graphene and Mos2 monolayer, the equilibrium configuration and the binding energy of the flakes, as well as the effects of substrate on the electronic structure have been investigated. While the cohesive and formation energies and HOMO-LUMO gaps of nanoflakes with their edges passivated by hydrogen display clear size, shape and edge geometry dependencies, they are rather dispersed in bare nanoflakes. The binding of phosphorene nanoflakes to two-dimensional (2D) phosphorene, graphene and MoS2 monolayers is generally weak and originate from van der Waals interaction. Accordingly, when supported by these monolayers, the electronic structure of free-standing nanoflakes can be preserved for critical applications. The ultra-small sized nanomaterials are important for basic functional components of future nanoelectronics, spintronics and sensor devices. In this study, based on first-principles density functional theory, the free-standing and supported nanoflakes of bare and hydrogen saturated black and blue phosphorene of diverse size and shape have been investigated. Cohesion, formation energy, thermal stability and electronic structure of these nanoflakes have been revealed. For nanoflakes supported by specific substrates, such as phosphorene, graphene and Mos2 monolayer, the equilibrium configuration and the binding energy of the flakes, as well as the effects of substrate on the electronic structure have been investigated. While the cohesive and formation energies and HOMO-LUMO gaps of nanoflakes with their edges passivated by hydrogen display clear size, shape and edge geometry dependencies, they are rather dispersed in bare nanoflakes. The binding of phosphorene nanoflakes to two-dimensional (2D) phosphorene, graphene and MoS2 monolayers is generally weak and originate from van der Waals interaction. Accordingly, when supported by these monolayers, the electronic structure of free-standing nanoflakes can be preserved for critical applications.Item Open Access Functional carbon and silicon monolayers in biphenylene network(American Chemical Society, 2022-06-28) Gorkan, T.; Çallıoǧlu, Şafak; Demirci, S.; Aktürk, E.; Ciraci, S.We investigated the effects of vacancy, void, substitutional impurity, isolated adsorption of selected adatoms, and their patterned coverage on the physical and chemical properties of metallic carbon and silicon monolayers in a biphenylene network. These monolayers can acquire diverse electronic and magnetic properties to become more functional depending on the repeating symmetry, size of the point defects, and on the type of adsorbed adatoms. While a carbon monovacancy attains a local magnetic moment, its void can display closed edge states with interesting physical effects. Adsorbed light-transition or rare-earth metal atoms attribute magnetism to these monolayers. The opening of a gap in the metallic density of states, which depends on the pattern and density of adsorbed hydrogen, oxygen, and carbon adatoms, can be used as the band gap engineering of these two-dimensional materials. The energy barriers against the passage of oxygen atoms through the centers of hexagon and octagon rings are investigated, and the coating of the active surfaces with carbon monolayers is exploited as a means of protection against oxidation. We showed that the repulsive forces exerting even at distant separations between two parallel, hydrogenated carbon monolayers in a biphenylene network can lead to the superlow friction features in their sliding motion. All these results obtained from the calculations using the density functional theory herald critical applications.Item Open Access Functionalization of single-layer nitrogene by vacancy, adatoms, and molecules(American Chemical Society, 2017-02) Kadioglu, Y.; Ü. Aktürk, O.; Aktürk, E.; Çıracı, SalimDespite its strong N2 molecule, recent studies have shown that nitrogen, the lightest group V element, remains stable in the free-standing, single-layer buckled honeycomb structure with two-dimensional (2D) hexagonal lattice. This structure is called nitrogene and is predicted to be a nonmagnetic, wide band gap semiconductor or insulator. In this paper, we investigated the formation of a single vacancy, as well as the adsorption of selected single adatoms and molecules on 2D nitrogene, using the supercell method within the density functional theory. Through the consideration of large supercells, the couplings between adjacent vacancies and adspecies are minimized; hence, the results are taken to represent single, isolated defect and adspecies. We found that a single vacancy contributes a local magnetic moment and filled and empty localized gap states at low temperature but is prone to instability due to thermal excitations. Adatoms are bound to the surface of nitrogene and form localized gap states contributing a diversity of electronic and magnetic properties. Adsorption of adatoms, such as B, C, Si, and N, however, give rise to local and strong reconstruction in nitrogene in their close proximity. Notably, a N adatom forms a strong N2 molecule by removing one N atom from nitrogene and leaves a vacancy behind. Conversely, the interactions between selected molecules, such as H2, O2, H2O, and N2, and the surface of nitrogene are rather weak and do not induce any change in the physical properties. However, H2 and O2 can be dissociated at the edges of a nitrogene flake and concomitantly can remove host N atoms to form NH2 and NO2 molecules. Calculated properties of adatoms adsorbed to nitrogene differ dramatically from the properties of those adsorbed to single-layer structures of other group V elements.Item Open Access Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, SalimPotential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).Item Open Access Glycine self-assembled on graphene enhances the solar absorbance performance(Elsevier, 2019) Ersan, F.; Aktürk, E.; Çıracı, SalimDespite its high solar absorbance and surface coating abilities, pristine graphene as a semimetal is not promising for photovoltaic applications. In this study, we predict that Glycine (Gly), an amino acid, which is normally bound to pristine graphene by a weak van der Waals attraction, can form an organic coating durable to ambient condition when adsorbed on vacancy patterned graphene surface. Moreover, adsorbed Gly coating induces metal-insulator transition and concomitantly increases the solar absorbance of pristine graphene more than three times. This way, graphene attain critical functionalities to be used in solar energy and photovoltaic applications.Despite its high solar absorbance and surface coating abilities, pristine graphene as a semimetal is not promising for photovoltaic applications. In this study, we predict that Glycine (Gly), an amino acid, which is normally bound to pristine graphene by a weak van der Waals attraction, can form an organic coating durable to ambient condition when adsorbed on vacancy patterned graphene surface. Moreover, adsorbed Gly coating induces metal-insulator transition and concomitantly increases the solar absorbance of pristine graphene more than three times. This way, graphene attain critical functionalities to be used in solar energy and photovoltaic applications.Item Open Access High-capacity hydrogen storage by metallized graphene(AIP Publishing, 2008) Ataca, C.; Aktürk, E.; Çıracı, Salim; Ustunel H.First-principles plane wave calculations predict that Li can be adsorbed on graphene forming a uniform and stable coverage on both sides. A significant part of the electronic charge of the Li 2s orbital is donated to graphene and is accommodated by its distorted π* -bands. As a result, semimetallic graphene and semiconducting graphene ribbons change into good metals. It is even more remarkable that Li covered graphene can serve as a high-capacity hydrogen storage medium with each adsorbed Li absorbing up to four H2 molecules amounting to a gravimetric density of 12.8 wt %.Item Open Access Hydrogen storage of calcium atoms adsorbed on graphene: First-principles plane wave calculations(American Physical Society, 2009) Ataca, C.; Aktürk, E.; Çıracı, SalimBased on first-principles plane wave calculations, we showed that Ca adsorbed on graphene can serve as a high-capacity hydrogen storage medium, which can be recycled by operations at room temperature. Ca is chemisorbed by donating part of its 4s charge to the empty π∗ band of graphene. At the end the adsorbed Ca atom becomes positively charged and the semimetallic graphene changes into a metallic state. While each of the adsorbed Ca atoms forming the (4×4) pattern on the graphene can absorb up to five H2 molecules, hydrogen storage capacity can be increased to 8.4 wt % by adsorbing Ca to both sides of graphene and by increasing the coverage to form the (2×2) pattern. Clustering of Ca atoms is hindered by the repulsive Coulomb interaction between charged Ca atoms.