Geometrical indications of adsorbed hydrogen atoms on graphite producing starlike and ellipsoidal features in scanning tunneling microscopy images Mohammad Khazaei, Ahmad Ranjbar, Mohammad Saeed Bahramy, Hiroshi Mizuseki, and Yoshiyuki Kawazoe Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan (Dated: January 24, 2009) Recent scanning tunneling spectroscopy (STM) experiments display images with star and ellip- soidal like features resulting from unique geometrical arrangements of a few adsorbed hydrogen atoms on graphite. Based on first-principles STM simulations, we propose a new model with three 9 hydrogen atoms adsorbed on the graphene sheet in the shape of an equilateral triangle with a 0 hexagon ring surrounded inside. The model reproduces the experimentally observed starlike STM 0 patterns. Additionally, we confirm that an ortho-hydrogen pair is the configuration corresponding 2 to the ellipsoidal images. These calculations reveal that when the hydrogen pairs are in the same n orientation, theyare energetically more stable. a J Hydrogenadsorptionon graphite surface has been the containing 96 carbon and appropriate number of hydro- 4 2 subject of many theoretical and experimental studies, genatoms (two, three, or four). The correspondingBril- due to its fundamental importance in both science and louinZoneissampledbya5×5×1Monkhorst-Packmesh. ] technology [1, 2, 3, 4, 5, 6, 7, 8]. The STM technique is Thestructuresarefullyoptimizeduntilthemagnitudeof i c knownasoneofthemostpowerfulexperimentaltoolsfor force on each ion becomes less than 0.04 eV/˚A. There is s analyzingsurfaces. Throughthistechnique,forexample, currently no simple way to apply the electric field in the - l Hornekær et al. considered the atomic structures of var- VASP calculations. Accordingly, we use the SIESTA, r t ious types of H(D) clusters on graphite [6, 7]. In their which is a DFT code with localized basis sets [10], for m latestexperiments,theyidentifiedtwotypesofclustersas the simulation of STM images. We performed a new set . themostabundanthydrogenspeciesongraphitesurface. of spin-polarized single-point energy calculations for the t a The STM images of these clusters exhibit star and ellip- structures, previously optimized by VASP, in the same m soidallikefeatures,seeFIG.1(a)[8]. Theformerincludes leveloftheory(DFT/PBE)butatdefiniteelectricfields. - six brightspots, three ofwhich arerelatively largerthan A full description of our methodology for STM calcula- d the others [8]. Considering the STM images observed at tions can be found in Ref. [11, 12, 13]. n variousbiasvoltages,Hornekæretal. concludedthatthe o To identify the geometrical positions of hydrogen c starlike feature originatesfrom a particular arrangement atoms on graphite producing the starlike STM image, [ of three or four hydrogen atoms adsorbed on graphite wefirstexaminemodelswithfourHatoms(labeledasS1 surface [8]. For the ellipsoidal STM feature, using a 2 and S2 ) and three H atoms (labeled as S3 and S4), pro- set of STM simulations, Hornekær et al. proposed that v posedby Hornekæret al. [8], see FIG. 1(b). Figures 1(c- 3 it should represent a pair of hydrogen atoms adsorbed f)showtheSTMimagescomputedforS1,S2,S3,andS4 1 on two adjacent carbon atoms, so-called ortho-hydrogen under an electric field of 0.5 V/˚A. From FIGs. 1(c-f), it 4 pair. Although, the computed STM image by Hornekær is evident that none of the above models can reproduce 1 et al. appears to be in agreement with experiment, the . theexperimentallyobservedSTMpattern. Interestingly, 2 authors have not clarified how a pair of hydrogen atoms the computed STM images of S1 and S2 with four hy- 1 with H-H distance ∼2.1 ˚A can produce an elongated- drogen atoms have only three bright spots while the S3 8 ellipsoidal feature with a length of ∼7.0 ˚A [6, 8]. In 0 structure with three hydrogen atoms shows four bright the present work we propose a model whose computed : spots in its STM pattern. This can be attributed to the v STM image perfectly matches with experimentally ob- fact that the computed current is substantially propor- i served starlike STM images. Additionally, we confirm X tional to the electron tunneling and LDOS. For S1 and that the ortho-hydrogenpair is the configuration for the r S2, our calculations show almost no LDOS at left turn- a ellipsoidal images and explain how these pairs produced ing points of the central hydrogen atom. As a result, the elongated features in STM images. We further in- the emitted current from the central hydrogen atom be- vestigatetheinteractionofhydrogenpairstogether. Our comes negligibly small. On the other hand, for S3, both calculationsshowthatwhenthe pairsareorientedinthe the electron tunneling and LDOS turn out to be signifi- same direction, they are energetically more stable. cantlyhigherforHatomsandtheircentralcarbonatom. Accordingly,thesefouratomscontributesubstantiallyto The electronic structure calculations are carried out the total emitted current, as can be seen in FIG. 1(e). withinthecontextofdensityfunctionaltheory(DFT)us- ing the spin-polarized Perdew−Burke−Ernzerhof(PBE) We propose a new model S5, in which three hydrogen exchange-correlation functionals and the projected aug- atomsaresymmetricallyplacedonthe graphenesheetin mented wave (PAW) method, as implemented in the anequilateraltriangleencompassingacompletehexagon VASP code [9]. The adsorbed hydrogen atoms on ring of carbon atoms, see FIG. 1(g). The configuration graphite surfaces are simulated using a graphene sheet S5 has a large magnetic moment of 3µ . B 2 FIG. 2: (Color online) The geometrical configuration of S6 and its possible transformation pathway toS5. tion of S5, the centers of the larger (smaller) spots turn out to be on the hydrogen atoms (the carbon sites indi- catedbyblackspotsinFIG.1(g)). Inagreementwithex- perimental results reported by Hornekær et al., our sim- ulations demonstrate that when the strength of applied voltagesdecreases(here from 0.7to 0.3 V/˚A), the inten- sityofthe smallspots reduces. To explainthe lowinten- sityofsmallspotsatlowvoltages,wehaveconsideredthe patternsoftotalLDOSandtotaltunnelingprobabilityof S5 (not shown here). These results show that the LDOS patternsare almostthe sameunder variousappliedvolt- ages. At high-applied voltage 0.7 V/˚A, the tunneling probabilitiesofelectronsfrombothhydrogenandcarbon atoms are large. At lowervoltages,0.3 and 0.5 V/˚A, the tunneling probability of electrons from hydrogen atoms is relatively larger than that from carbon atoms. Such a difference can be attributed to the fact that the ad- sorbed hydrogen atoms create dipole moments on the graphene surface whereby the effective potential around thehydrogenatomsdecreasesandconsequently,thetun- nelingprobabilityofelectronsfromthem increases. This explains the reason for high electron emission from hy- drogenatomsinallrangesofappliedvoltages. Itisworth mentioning that further calculations using a larger unit FIG. 1: (Color online) (a) Experimental STM image of ad- cell with a side length of ∼35˚A reveals no changein the sorbed hydrogen atoms on graphite with starlike and ellip- computed STM images and magnetic properties of S5. soidal features [8]. The magnified starlike feature is shown To compare the stability of S5 with other structures, in the inset, (b) the experimentally proposed configurations we summarize in Table I, the respective adsorption en- fortheadsorbedhydrogenatomsproducingthestarlikeSTM ergy values of hydrogen atoms for optimized S3, S4, S5 images [8], and (c-f) the computed STM images for S1-S4 andS6configurations(thelatterisillustratedinFIG.2). at 0.5 V/˚A, respectively. Red (blue) color denotes the high- The adsorptionenergyis definedasE =E − est(lowest)intensityoftheemittedcurrents. (g)Geometrical a graphene+nH structureofS5,and(h-j)itscorrespondingSTMimagescom- Egraphene−nEH whereEgraphene+nH,Egraphene andEH puted at 0.7, 0.5, and 0.3 V/˚A,respectively. arethe totalenergies obtainedfor a graphenesheet with n adsorbed hydrogen atoms, a perfect graphene sheet andanisolatedHatom,respectively. Overallcomparison showsthatthemoststablestructures,S3andS5,areen- The spin-polarizedcalculationshowsthattotalenergy ergeticallyisomeric. However,sincetheSTMimageofS3 is approximately 0.1 eV less than that estimated by non is totally different from that observed in experiment, S5 spin-polarized calculation. Therefore the S5 structure is appears to be practically preferable to S3. This implies expectedtobemagneticevenatroomtemperature. Fig- that there should be a post-adsorption mechanism af- ures 1(h-j) showthe STM imagesof S5 computed at0.7, fecting the structure of initially adsorbed hydrogenclus- 0.5, and 0.3 V/˚A, respectively. The computed STM im- tersongraphite,otherwise,S3andS5shouldstatistically ages all show a starlike feature similar to that observed havehadthesamepopulation,inpractice. Thus,theex- inexperiment. Therearesixbrightspots,threeofwhich perimental conditions, within which the above STM im- look relatively larger than the other three. Comparing ages have been observed, need to be carefully taken into the position of the spots and the geometrical configura- account. In the experiment, S5 is observed in samples 3 TABLE I: Adsorption energy (Ea) values obtained for the triatomicconfigurations,S3-S6,andthedimerconfigurations, D1-D7. structures Ea(eV) structures Ea (eV) S3 -2.63 D1 -2.8 S4 -2.61 D2 -1.65 S5 -2.62 D3 -2.75 S6 -2.43 D4 -1.59 D5 -2.18 D6 -1.67 D7 -1.83 which are annealed to 525K after a heavy deposition of H(D)atomsonthesurface. SincetheutilizedH(D)beam isextremelyhot(1600-2200K),hydrogenclusters,invar- ious configurations including S1-S6, are created [7, 8]. However, by annealing the sample, many of these clus- ters are evaporated from the surface or change to other FIG. 3: (Color online) (a) the geometrical structure of D1- configurations. A statistical study by Hornekær et al. D7, (b-h) the respective STM images computed for D1-D7. revealsthat, the relativeabundance oftwo specific H(D) (i) The projection of STM image of D1 onto its geometrical species with starlike and ellipsoidal STM features dom- configuration. inantly increases, as the sample is further annealed to 570 K [8]. The process, described above, suggests that are the eigenmodes of initial state (IS) and transition after annealing the sample, many H(D) species initially state (TS), and as pointed out earlier ∆E⋆ is the en- depositedonthesurfacetransformintoS5configuration. ergy difference between TS and IS after including zero Forsuchatransition,basically,specieswithsimilarsym- point energy correction. On this basis, the respective metry but higher energy such as S6 are preferred. valuesofr at300Kand500KareexpectedtobeS6→A: A possible transition pathway from S6 to S5 is de- 11.2×104s−1 and3.1×108s−1,A→B:6.0×10−2s−1 and picted in FIG. 2. In this picture, The diffusion reactions S6→A, A→B, and B→S5 are assumed to pass through 4.3×104s−1,andB→S5: 2.7×10−2s−1,and2.8×104s−1. the barriers ∆E⋆ = E⋆ − E⋆ , ∆E⋆ = E⋆ − E⋆ These results clearly verify that the proposed transition and ∆E⋆ = E⋆ 1 −E⋆TSw1hoseSsa6ddle p2oints aTrSe2at thAe processispracticallyveryfeasible. Itisnecessarytonote 3 TS2 B thatS5 maynotbe the moststable configurationfor ad- transition states, TS1, TS2, and TS3, respectively. Note sorption of three hydrogen atoms on graphene. Rather, that, above all E⋆ terms include the zero-point energy it is a metastable structure made in a particular experi- correction. To find the correct transition paths and the mental conditions. corresponding energy barriers, we use the nudged elas- As mentioned earlier, the experimentally observed tic band method [15]. As a criterion for determining STM images include patterns with elongated-ellipsoidal thesaddlepoints,the phononeigenmodesofeachTSare features [8]. Hornekær et al. proposed that such fea- examined so that they have one and just one imaginary tures originate from a pair of H atoms, adsorbed on two frequency. Accordingly, ∆E⋆, ∆E⋆ and ∆E⋆, are ob- 1 2 3 adjacent carbon atoms of graphite [6]. Although their tained to be 0.48 eV, 0.84 eV and 0.86 eV, respectively. model, here labeled as D1, appears to reproduce the The maximum influence of vibrationalzero-pointenergy same ellipsoidal STM features, their calculations fail to correction on barriers is less than 0.12 eV. For the sake explain,howapairofhydrogenatomswithH-Hdistance of comparison, we have also calculated the energy bar- of∼2.1˚Acanproduce a brightellipsoidalfeature as long rier for desorbing each of hydrogen atoms, supposed to bedisplacedthroughS6→A,A→B,andB→S5reactions. as∼7.0˚A[8]. Toanswerthisquestion,wehavecomputed theSTMimageforD1andotherpossibleHdimers. The The respective desorption energies are 0.67, 1.0, and 1.2 structureofdimersandtheircorrespondingSTMimages eV. Thus by annealing, the hydrogen atoms in S6 pre- areillustratedinFIG.3. Additionally,inTableI,wehave fer to diffuse to S5 through the process described above summarizedthevaluesofadsorptionenergies. According rather than to be desorbed from the graphene surface. to FIG. 3, each dimer has a unique STM pattern. Re- To further asses the possibility of above reactions at assuringly, D1, is the only dimer structure whose STM different temperatures, we haveestimated the rate of re- image represents ellipsoidal feature, similar to that ob- actionsat300Kand500Kwithinthetransition-statethe- servedinexperiment. Interestingly,thebrightellipsoidal ory [16]. In this approach,the rate of reactionis defined pattern obtained in our STM calculation for D1 has al- asr=ΠΠ′ii11−−eexxpp((−−h¯h¯ωωiTiISS//kkBBTT))exp(cid:16)∆kBET⋆(cid:17)whereωiIS andωiTS most the same experimental length ∼7.0˚A. For the sake 4 cell 34.08˚A×14.76 ˚A (34.08˚A×29.52 ˚A), containing 192 (384) C atoms. The corresponding values of adsorption energies are summarized in Table II. The results clearly indicatethatdimerswithsamedirectionbecomeenerget- ically more stable, as they get closer to each other. On the other hand, the disorientation of H dimers results in an increase in the surface energy and, hence, in insta- bility of whole structure. Consequently, the adsorbed H dimers prefer to diffuse ongraphenesurface so that they can stay in the same directions. FIG. 4: (Color online) The relative positions of (a) two hy- In conclusion based on STM image calculations, we drogen dimers and (b) three hydrogen dimers on a graphene have identified the geometrical configurations of the sheet. The highlighted C-C bond denoted by numbers 1-9 most abundant species of adsorbed hydrogen atoms on indicate theposition of displaced dimer. graphite after a heavy dosing ofH(D) atoms. The struc- TABLE II: Adsorption energies Ea of hydrogen dimers on of clarity, in FIG. 3(i) we have projected the D1’s STM graphene sheets shown in FIGs. 4(a) and (b) imageontoitsgeometricalstructure. Evidently,notonly Position of Ea of two dimers Ea of three dimers the hydrogen atoms but also their neighboring carbon displaced dimer (eV) (eV) atoms contribute significantly to the total STM current. 1 -5.58 -8.18 In other words, the creation of D1 on graphene changes the electronic structure of its surrounding carbon atoms 2 -5.49 -8.21 such that their corresponding LDOS values become sig- 3 -5.69 -8.19 nificantly large. Consequently, they contribute substan- 4 -5.54 -8.18 tially to the STM current. D1 is nonmagnetic system 5 -5.66 -8.22 andaccordingtoTableI,itis energeticallythe moststa- 6 -5.39 -8.16 ble configuration in comparison to other dimer models. 7 -6.17 -8.12 ExperimentalresultsalsoshowthatD1isthemostabun- 8 -5.64 -8.27 dantly formed configuration. 9 -5.71 -8.18 Interestingly, it appears that many of the H dimers absorbed on graphite, seems to be oriented in the same direction. This can be clearly seen in FIG. 1(a), see the bright spots discriminated by the white circles. To elab- tures were shown to have two and three hydrogenatoms orate on this observation, we have carried out a set of with STM images having elongated-ellipsoidal and star- calculations in which the adsorptionenergy is calculated likefeatures,respectively. Theformer(latter)turnedout for two andthree H dimers when they havedifferent ori- to be nonmagnetic (strongly magnetic). In the case of entations and distances in respect to each other. 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