Effect of the alkali metal(Li,Na,K)substitution on the geometric,electronic and optical properties of the smallest diamondoid:First principles calculations

Sriprajak Krongsuk*,Nikorn Shinsuphan,Vittaya Amornkitbumrung

1Department of Physics,Faculty of Science and Integrated Nanotechnology Research Center,Khon Kaen University,Khon Kaen 40002,Thailand

2Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage,Khon Kaen University,Khon Kaen 40002,Thailand

3Thailand Center of Excellence in Physics(ThEP),Commission on Higher Education,Bangkok 10400,Thailand

Keywords:Functionalization Electronic structure Molecular stability Bathochromic shift DFT calculations

A B S T R A C T In this study we employed the B3LYP/6-311++G(d,p)method combined with the CIS/6-311++G(d,p)calculationtoinvestigatetheeffectsofthetypeandthenumberofalkalimetalatoms(Li,Na,K)onthegeometric,electronic,and optical properties of alkali metals substituted into adamantanes.Substituting alkali metal(Li,Na,K)atoms caused significant changes in the electronic and optical properties of adamantane.The Ad-1Li,Ad-1Na,and Ad-1K structures showed a dramatically decreased energy gap and ionization potential,while adding more alkali metal atoms slightly decreased these properties.Substituting more alkali metals led to a shift in the maximum absorption wavelength from the visible to the infrared region,depending on the type of alkali metal atom substituted.The magnitude of shift occurred inthe followingorder:Li＜Na＜K.These characteristics suggest the possibility of tunable electronic structures of this material for optoelectronic device applications.

1.Introduction

Diamondoids are organic compounds thathave linked carbon atoms that forma cagelikestructure terminatingwith hydrogenatomsattheir surface.The smallest diamondoid is adamantane(C10H16)consisting of 10 carbon atoms arranged as a single cage crystalline subunit,surrounded by 16 hydrogen atoms(Fig.1a).Larger diamondoids can be created by connectingmore diamond cages and they are categorized according to the number of diamond cages.These molecules have been ofgreatinterestinboththeoreticalandexperimentalresearchersdueto their important role in nanotechnology as molecular building blocks,drug-delivery and medicine,and nanoelectronic devices[1,2].They have unique physical and chemical properties,for examples,they exhibit higher melting points,shape dependent optical absorption,ultraviolet photoluminescence,and negative electron affinity[1,3,4].Therefore,many studies have addressed functionalizing different classes of diamondoids[5-11].Functionalization can be accomplished by substituting other atoms for the carbon or hydrogen atoms.It can also be done by substituting chemical functional groups for the hydrogen atoms.This leads to improved structural,electronic,and optical properties[10,12,13].Due to the wide variety of functionalized diamondoids with their unique physicochemical properties,they have been widely investigated in the recent years for nanotechnology applications such as a nanomaterial for N2gas capture[14],optoelectronic devices[15-18],energy storage devices[19],and in medicine[13].

Fig.1.Molecular structure(a)the pristine adamantane(Ad),(b)Ad-1Li(Na,K),(c)Ad-2Li(Na,K),(d)Ad-3Li(Na,K),and(e)Ad-4Li(Na,K).All alkali metal atoms represent in pink.

Among the diamondoid family,the adamantane molecule(Ad)has been widely studied by theoretical and experimental researchers due to its unique properties which include negative electron affinity(NEA),sp3hybridization,its high degree of symmetry as a tetrahedral(Td)point group,a high melting point and shape dependent electronic and optical gaps[1,5-11].It is well established that adamantane with a hydrogen terminated surface has a high negative electron affinity.So,electrons can readily escape from its surface[20,21].Therefore,it canbepresumedthatdopingusingBandNatomscanenhancetheelectron-emittingpropertiesofdiamondoids[22].Additionally,thetheoretical study of Garcia et al.[23]revealed that the B and N doping of adamantane showed a very highly stable and large bulk modulus.Diamondoids with functional groups such as a hydroxyl(OH),amine(NH2)and thiol(SH)also received great interest in theoretical and experimental studies[5,13,16-20]because these functional groups can strongly bind to targeted molecules or substrates.Among these functional groups,the thiol group is commonly used to attach diamondoids to metal surfaces[24].This infers a means of binding diamondoids to various entities such as biomolecules or organic semiconductors.Landt et al.[24]studied the absorption properties of thiolated diamondoids using both experimental and theoretical methods.They found that the optical properties of these materials are dependent on the functionalized position and the size of the diamondoids.Their studies revealed that theoptical gapof adamantane-1-thiol is～0.6 eV lower than pristine adamantane and its UV luminescence is quenched.Na substituted adamantane was investigated by Hamadanian et al.[25].This study found that the electronic band gap of functionalized adamantane was reduced with an increasing number of Na atoms.It also became semi-metallic or metallic.The alkali metal functionalized adamantanes have been investigated by means of first principles calculations[8,16,17].Xue and Mansoori[8]revealed that the Na substituted adamantane has considerable conductance and interesting electronic properties.Substituting one tertiary hydrogen atom of adamantane with Li,Na,and K atoms was investigated by Wu and co-workers[16].Their study revealed that the maximum absorption wavelengths of these molecules were in the visible region and showed a red shift trendfromtheLitoKcomplex.Songetal.[17]focusedonLiandNasubstitution for different hydrogen atoms of adamantane.Their results revealed that the first hyperpolarizability of Ad-nLi(n=1-4)decreased with increasing numbers of Li atoms,while the first hyperpolarizability of Ad-nNa showed the reverse trend.However,there are still some questionsrequiringfurtherinvestigation.Forexample,howdotheelectron emission properties change with increasing numbers of alkali metal atoms?How much are the electronic structure and optical propertiesofadamantaneinfluencedbythetypeofalkalimetalusedforsubstitution?Furthermore,thegeometric,electronic,andopticalproperties of K-substitutedadamantane have notbeen fullyinvestigated.Forthese reasons,we have focused on alkali metal(Li,Na,K)substituted adamantanes by means of first principles calculations.One to four tertiary hydrogen atoms of adamantane were replaced by Li,Na,or K atoms to investigate the effect of alkali metal atom substitution on the electronic structure and optical properties of adamantane.This functionalization does not only improve the structural and electronic properties of adamantane,but also provide for an efficient way to fabricate high performance nonlinear optical materials.

2.Computational Details

Firstly,we performed a geometric optimization of pristine adamantane(Ad)using two levels of theoretical methods including densityfunctionaltheory(DFT)[26]andthesecondM?ller-Plessetperturbation(MP2)methods[27].For DFT calculations,we employed SWVN,BLYP,and B3LYP to describe the exchange-correlation term.Additionally,two types of basis sets including 6-311++G(d,p)and augcc-pVDZ were selected for this calculation since they properly describe the molecular structure and surface nature of delocalized electrons in carbon based materials[28].Geometric parameters and electronic properties obtained from the calculation methods are summarized in Table 1.Finally,the B3LYP/6-311++G(d,p)method was chosen and used throughout this study because it is quite effective in calculating satisfactory geometries and electronic properties at a relatively small computational cost[33].A detailed discussion is given in Section 3.1.

For the functionalization of adamantane,we modified its structure by substituting 1 to 4 tertiary hydrogen atoms with alkali metal atoms(Li,Na,K).This led to the molecular structure of C10H16?nMngiven in Fig.1b-e,respectively,forLi,NaandK.Here,Mrepresentsthetypeofalkali metal(Li,Na,or K)and n is thenumberof alkali metalatom varyingfrom 1 to 4.These modified structures are referred to as Ad-nL,Ad-nNa,and Ad-nK,respectively.Next,all these functionalized structures were subjected to geometric optimization followed by calculation of their electronic structure and enthalpy using the B3LYP/6-311++G(d,p)method.Theenthalpyofchemicalreaction(ΔH)wasdefinedforthefollowing reaction[25]:

Table 1 Geometric and electronic properties of the pristine adamantane obtained from various levels of theory and basis sets.C′represents a carbon atom connected to a tertiary hydrogen atom(H′)of adamantane

Table 2 Structural properties of the alkali metals(Li,Na,K)substituted into adamantanes obtained from the B3LYP/6-311++G(d,p)calculations.The C′and H′refer to tertiary carbon and hydrogen atoms,respectively,and M an alkali metal atom(Li,Na,K).C1,C2,and C3represent the first,second and third carbon atoms that connected to two hydrogen atoms

The calculated geometric parameters,electronic properties and enthalpies for the chemical reaction for all of these structures are summarized in Tables 2 and 3.

Table 3 Molecular properties of the pristine adamantane and those functionalizedwith Li,Na,and K atoms.Ionization potential(IP),electron affinities(EA),energy gap(Eg),chemical hardness(η),electronegativity(χ),dipolemoment(μ)andenthalpyofchemicalreaction(ΔH)were calculated using the B3LYP/6-31++G(d,p)level of theory

To investigate the optical properties of pristine adamantane and alkali-metal(Li,Na,K)substituted adamantane,we employed the CIS method[34]with the 6-311++G(d,p)basis set rather than the B3LYP method because it gives an accurate excitation energy,especially for a single excitation.All calculations were performed using the Gaussian 09softwarepackage[35]ontheLinuxPCclustersatthePhysicsDepartment,Khon Kaen University.

3.Results and Discussion

3.1.Geometric and electronic properties of pristine adamantane

Table 1 shows the optimized geometric parameters and electronic propertiesofpristineadamantaneobtainedfromthevarioustheoretical methods.C′and H′are the tertiary carbon and hydrogen atoms of adamantane,respectively.It can be clearly seen that the average C--C bond length and the C--C--C bond angle obtained from all methods are in good agreement with the experimental values(C--C=0.154±0.001 nm,C--C--C=109.5°± 1.5°)[29],while the bond lengths of C--H and C′--H′and the angle of H--C--H are slightly different.Additionally,among all methods,the B3LYP and MP2 methods yielded a C--C bond length very close to the experimental value of 0.154 nm.It is notable that geometric optimization at the MP2 level using 6-311++G(d,p)and aug-cc-pVDZ basis sets required time consuming calculations.

Furthermore,the electronic properties including the energy gap(Eg),ionization potential(IP),and electron affinity(EA)were calculated and are compared in Table 1.The Egis the energy difference between thehighestoccupiedmolecularorbital(HOMO)andthelowestunoccupied molecular orbital(LUMO).EA and IP are defined as follows.

whereEneutralis thetotalenergyoftheneutralmoleculeinits optimized structure.Eanionand Ecationare the total energies of the corresponding anion and cation calculated in their respective optimized structures.Clearly,theelectronic properties are strongly dependentonthemethod and basis set used.The MP2 method yielded larger overestimated values of the Egand EA for the two basis sets compared with the experiment value(6.03 eV)[30]and QMC calculation(?0.13 eV)[32].Although the SVWN method gave better results for the Egand IP values,it failed to calculate the EA.Similarly,the BLYP method yielded the best results for Egand EA,but its results for the IP calculation were poor.The B3LYP method provided acceptable results for the Egand EA

calculations,and it yielded the best IP value.It was also found that the B3LYP/6-311++G(d,p)method yielded an IP of 9.23 eV and an EA of

?0.43 eV.These were better than for the B3LYP/aug-cc-pVDZ method.This result can be compared to an experimental IP value of 9.24 eV[31]and a calculated Egvalue(7.61 eV)obtained from the QMC method[32].ItcanbeclearlyseenthattheB3LYP/6-311++G(d,p)methodprovided reasonable geometric and electronic properties that are close to the experimental values.Therefore,the B3LYP/6-311++G(d,p)method was selected for use in further calculations.

3.2.Structuralpropertiesofalkali metal(Li,Na,K)substituted adamantane

Table 2 shows the structural properties of alkali metal(Li,Na,K)substituted adamantanesas a function of the number of substitute atoms(n=1-4).Obviously,substituting one to three tertiary H atoms with Li(Na,K)atoms causes elongation of the C--C,(C--C′)and C′--H′bond lengths and distortion of the M---C--C1,M--C--C2and M--C--C3angles.Consequently,these molecules had lower symmetry groups(C3Vand C2V)compared to the pristine adamantane structure(Td).When four tertiary H atoms were substituted with alkali metal atoms,the substituted the C--C′bond lengths shrank and the three M--C--C bondangles remained unchanged,leadingto itshighgeometric symmetry(Td).The optimized structures of Ad,Ad-4Li,Ad-4Na,and Ad-4K are presented in Fig.2.It can be seen that the C--M bond length andtheM--Mdistanceincreasedwiththetypeandthenumberofalkali metal atoms.These structural changes can be ordered as follows:Li＜Na＜K,respectively.According to the DFT study of Hamadanian et al.[25],our calculations show consistency with the geometric parameters ofNasubstitutedadamantane,suggestingreliabilityofthemethodology used in this study.

3.3.Electronic properties and molecular stability

Table 3 shows the electronic properties and molecular stability of pristine adamantane and alkali metal(Li,Na,K)substituted adamantanes.Their IP,EA,and Egvalues were calculated as defined in Section 3.1.Additionally,the electronegativity(χ)and the chemical hardness(η)were obtained from the EA and IP values as follows:

Electronegativity is a chemical property that describes the tendency of an atom to attract electrons.Chemical hardness describes the resistance to charge transfer.It can be clearly seen from Table 3 that the IP and Egvalues for all substituted adamantanes decreased with an increasing number of alkali metal atoms.These properties are greatly reducedwhenoneLi(orNa,K)issubstituted.TheIPvaluesforAd-1Li,Ad-1Na,andAd-1Kstructuresdecreasedby41%,44%,and52%,respectively.Similarly,the Egvalues for Ad-1Li,Ad-1Na,and Ad-1K structures decreased by 66%,70%,and 76%,respectively.However,substituting more Li(Na or K)atoms caused a slight decrease in both the IP and Egvalues.The magnitude of change in these properties was:Li＜Na＜K.This result suggests that substituting one tertiary H-atom with Li(Na or K)atom has a significant effect on the electronic properties of the adamantanemolecule.ThenegativeEAvalueofpristineadamantaneindicates that it releases energy(is exothermic)to gain electrons to form an anion.Therefore,it is more likely to be an electron acceptor.Conversely,when alkali metal atoms(Li,Na,and K)are substituted into it,the EA value becomes positive and increases with further substitution of alkali metal atoms.This means that they are electron donors.The change in Egand EA as a function of the substituted alkali metal atom is displayed in Fig.3.A material with the highest positive EA(1.473 eV)and the lowest Eg(0.730 eV)of the Ad-4K structure is potentially good for use in nanoelectronic devices that operate using the mechanism of the charge transfer.The chemical hardness(η)values also decreased with an increasing number and size of substituted alkali metal atoms.The electronegativity(χ)values of the Li-Ad and the Na-Ad showed a decreasing trend with the number of substituted atom,but this is not the case of the K-Ad.The electronegativity values for all K substituted adamantanes are nearly the same with small variation.Clearly,substituting more alkali metal atoms does not have a significant effect on the electronegativity properties.The dipole moments(μ)describing the charge distribution of the functionalized adamantanes were examined and the results are presentedin Table 3.This property is closely relatedto molecularsymmetry.Itcan beclearly seenthat adamantanes in which one to three alkali metal atoms have been substituted had a lower molecular symmetry and consequent nonzero dipole moment due to their non-uniform charge distributions.Adamantane into which four alkali metal atoms were substituted had a higher symmetry due to the uniformity of charge distribution.It had a zero dipole moment as was observed in the pristine structure.

Fig.2.The optimized structures of(a)Ad,(b)Ad-4Li,(c)Ad-4Na,and(d)Ad-4K.

Fig.3.Plots of energy gap(Eg)and electron affinity(EA)of the alkali metal(Li,Na,K)substituted adamantanes as a function of the number of substitute alkali metal atoms.

To investigate the reaction energy of alkali metal substituted adamantanes,the enthalpy of chemical reaction(ΔH)was calculated and is given in Table 3.The enthalpies of all structures increased with thetype and the number of substituted alkali metalatoms and had positive values,suggesting that this functionalization is an endothermic process.This means that the alkali metal(Li,Na,K)substituted adamantanes were less stable than the pristine adamantane.The Ad-4K had an enthalpy of 0.227 eV,which was greater than that of the Ad-4Li(0.138 eV)or the Ad-4Na(0.195 eV),suggesting a more reactive molecule.Note that we presentthe structural stability of all alkali metal substituted adamantanes with respect to the pristinestructure in terms of the energy change in the chemical reaction(called enthalpy change).Enthalpyofformationreferstotheenergyinvolvedintheformationofa substancefrom its elements in their most stable forms.Higher enthalpy means heat had to be absorbed to form the substance,which makes the substance a higher-energy compound.High energy compounds tend to bemorereactive,thereforelessstable.However,thetheoreticalstudyof Masood Hamadanian et al.,[25]shows that the reaction energies of Nadoped adamantane compounds calculated in solid phase decrease faster.This means that these compounds are more stable in solid phase than in gas phase.In the aspect of optoelectronic device applications,it would be suitable for considering these compounds in solid phase.

To gain insight into its electronic structure,we generated the electronic density of states(DOS)between the occupied and unoccupied states for the pristine adamantane and the alkali metal(Li,Na,K)substituted adamantanes using a Gauss-Sum software package[36].TheseareasgiveninFig.4.TheDOSofpristineadamantanerepresented at the top of Fig.4(a)shows a wide band gap(7.224 eV)in which the Fermi energy is between the HOMO and LUMO states.This material can be categorized as an insulator.Substituting one or more Li atoms into the adamantane decreased the difference between the HOMO and LUMO states,leading to a narrower energy gap as presented in Fig.4(a).Similar results were observed for the cases of Na and K substitution as seen in Fig.4(b)and(c),respectively.This effect suggests that the electronic structure of these materials can be tuned so that they could have either semiconducting or conducting properties.

Fig.4.Total electronic density of states of the pristine adamantane and its functionalization by substitution with(a)Li,(b)Na,and(c)K atoms.

3.4.Optical properties of the alkali metal substituted adamantanes

Table 4 shows theoptical absorption properties of pristineand alkali metalsubstitutedadamantanes obtainedfromtheCIS/6-311++G(d,p)calculations.Thethree maintransitionstates andcorrespondingoscillator strength(f),wavelength,and excitation energy were used for all functionalized adamantanes.The absorption spectrum plots of these compounds are given in Fig.5.Pristine adamantane shows an absorption wavelength that is less than 200 nm with a maximum peak wavelength(λmax)of 146 nm.This lies in the ultraviolet region.However,adamantane substituted with one to four of the Li,Na,or K atoms showed shifts in the absorption spectra from the visible to infrared region,ranging from 500 nm to 2000 nm.The main peak moved to a higher wavelength,indicating the existence of a red-shift phenomenon(bathochromic shift).It can be clearly observed in Fig.5 that the bathochromic shift is strongly dependent on the alkali metal size in the following order:Li＜Na＜K.Furthermore,the excitation energies were dramatically reduced for the adamantane into which one alkali metal(Li,Na,K)atom was substituted.Adding more Li(Na,K)atoms showed slightly decreased excitation energies.These characteristics are useful in optoelectronic devices.This is because the absorption band gap involving promotion of electron transition from the ground state(S0)to the first excited state(S1)(or a lower level to a higherlevel usually from a molecular orbital-called HOMO(S0)→LUMO(S1)states)can be tuned.

Table 4 Absorption properties of pristine adamantane and adamantane into which Li,Na and K atoms were substituted obtained from CIS/6-311++G(d,p)calculations

Fig.5.Theabsorptionspectraofadamantanesintowhichalkalimetalatoms(Li,Na,K)had been substituted calculated using the CIS/6-311++G(d,p)method.

4.Conclusions

We employed B3LYP/6-311++G(d,p)calculations to investigate the geometric optimization,electronic properties,and molecular stability of adamantanesintowhichalkalimetals(Li,Na,K)weresubstituted.WealsoemployedtheCIS/6-311++G(d,p)methodtocalculatetheoptical properties of these molecules.In this study,we focused on substituting one to four tertiary hydrogen atoms of adamantane with Li,Na,and K atoms.Our results show that the geometric,electronic,and optical absorption properties of the alkali metal substituted adamantanes strongly depended on the type and number of alkali metal atoms placed in the material.The geometric structure of Ad-4Li,Ad-4Na,and Ad-4K shows higher symmetry(Td)compared with the otherstructures.Themagnitudeof themolecularstability of thesecompounds is ordered as follows:Ad-4Li＜Ad-4Na＜Ad-4K.Substituting with one alkali metal atom(Li,Na,or K)dramatically changed the energy gap and electron affinity,while adding further alkali metal atoms slightly affected these properties.The narrower energy gap and more positive electron affinity of the alkali metals substituted into adamantanes as a function of the number of alkali metal atoms suggest thatthesecanbetunableelectronicstructuresthatcanbechangedfrom semiconducting to conducting properties.The maximum absorption wavelengths of the Ad-1Li,Ad-1Na,and Ad-1K are in the visible spectrum.Substitutingmorealkalimetals shifts tothemaximumabsorption wavelength from the visible to infrared region(500 nm-2000 nm).The magnitudeofthebathochromicshiftstronglydependsonthesizeofthe alkali metal atom as the following order:Li＜Na＜K.

Acknowledgments

S.K.gratefullyacknowledgesfinancialsupportfrom theThailandResearch Fund and Khon Kaen University[Grant Number MRG5580165]and the Higher Education Research Promotion and National Research University Project of Thailand,Office of the Higher Education Commission,through the Advanced Functional Materials Center of Khon Kaen University,Nanotechnology Center(NANOTEC),NSTDA Ministry of Science and Technology,Thailand.N.S.gratefully acknowledges partial support from Thailand Center of Excellence in Physics(ThEP).The authors would like to thank Dr.Jeffrey Roy Johns for manuscript proofreading.