dft simulation and vibrational analysis of the ir and

DFT Simulation and Vibrational Analysis of the IR and Raman Spectraof a CdSe Quantum Dot Capped by Methylamine andTrimethylphosphine Oxide LigandsAhmed M. Abuelela,†,‡,⊥ Tarek A. Mohamed,†,§ and Oleg V. Prezhdo*,‡

†Department of Chemistry, Al-Azhar University, Nasr City, Cairo 11884, Egypt‡Department of Chemistry, University of Rochester, Rochester, New York 14642, United States

ABSTRACT: Surface effects and interactions with capping ligands stronglyinfluence the properties of semiconductor quantum dots (QDs), opening uppossibilities for new technologies, e.g. molecule sensing, and limiting the efficienciesof other applications, e.g. based on QD luminescence. By computing and analyzingin detail the infrared and Raman spectra of two ligands showing qualitativelydifferent bonding to the QD surface, we demonstrate that vibrational spectroscopyconstitutes a powerful tool for studying surface−ligand interactions due to its highsensitivity to changes in molecular structure. By forming a covalent bond with aCdSe QD, a single molecule of trimethylphosphine oxide (TMPO) exhibits a strongred-shift in the frequency of PO that transforms from a double to a single bond.In addition, interaction with the QD breaks TMPO’s C3v symmetry, splitting thesignals arising from the three methyl groups. The interaction of a methylaminemolecule with the CdSe QD is weaker and occurs via a coordination bond.Nevertheless, a strong blue-shift is seen in the frequency of the NH2 wagging mode,arising due to steric hindrance of this motion induced by the QD proximity. The theoretical predictions agree well with theavailable experimental data, establish the mechanisms for QD−ligand interactions, and provide specific guidelines for vibrationalanalysis of QD surfaces.

1. INTRODUCTIONIn recent years, quantum dots (QDs) have drawn the attentionof a broad scientific community, demonstrating a wide range ofapplications in biology and materials sciences. QDs have beenused in fluorescence labeling for biological analysis andimaging,1 light-emitting diodes,2 field-effect transistors,3

lasers,4,5 quantum emitter antennas,6 quantum informationprocessing,7,8 spintronics,9 thermo-power devices,10 solarenergy harvesting,11 thermal12 and photochemical13 catalysis,etc. The novel and unique QD properties arise from the smallparticle size. Quantum confinement effects allow one to tunelight absorption and emission wavelengths, direction of chargeand energy flow, photocatalytic activity, etc.Passivation of colloidal QD surface with organic ligands

constitutes a natural outcome of solution-phase synthesis.14

Surface passivation significantly influences QD optical proper-ties, which depend on the nature and degree of passivation.15

Photoinduced charge transfer to the QD surface is thought tobe the source of QD blinking16 and low yields of multipleexcitons.11b,17 Recent theoretical work demonstrated thatsurface ligands can strongly couple to the QD core18 andaffect the excited-state dynamics19 that are key for photovoltaicand photocatalytic applications. Similar observations werereported for surface and subsurface oxidation that occur duringcolloidal synthesis.20 The strong effects of surface conditions onthe physical and chemical properties of QDs motivate thestudies aimed at the understanding of the structural character-

istics of ligands bound to the QD surface and the degree ofligand interaction with the QD core.Vibrational spectroscopy provides a powerful tool for

investigating surface interactions in QD−ligand systems. Shiftsin the frequencies and intensities of vibrational peaks can beused to quantify the interaction strength for individualfunctional groups of the ligand. For instance, mid-infrared(IR) spectra were measured over a broad frequency range from500 to 4000 cm−1 for colloidal CdSe nanocrystals bound totrimethylphosphine oxide (TMPO) in tetrachloroethylene.21

Then, the TMPO ligand was exchanged with octylamine,octanethiol, and octanoic acid, and the vibrational analysis wasrepeated. The vibrational peaks corresponding to the ligandnormal modes showed significant changes upon QD binding.Fourier transform IR spectra were recorded over the samefrequency range for CdSe QDs capped with long-chain aliphaticprimary amines, including octadecylamine and hexadecyl-amine.22 Somewhat surprisingly, the chain length did notinfluence the investigated spectra.Despite the available experimental data on the vibrational

spectra of organic ligands bound to semiconductor QDs,21,22

the corresponding theoretical efforts are quite limited.21 Thegeometric and electronic structure of QD−ligand systems has

Received: April 5, 2012Revised: June 4, 2012Published: June 12, 2012

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achieved significantly more attention.18,23 The calculation ofthe vibrational spectra is much more time-consuming thanelectronic and geometric structure optimization, since itrequires normal-mode analysis involving second derivatives ofenergy with respect to nuclear displacements. Density func-tional theory (DFT) calculations are widely used to simulatethe vibrational spectra for molecules of relatively moderatesize.24 The calculations are greatly facilitated by the availabilityof analytic second derivative techniques,25 which reduce thecomputational cost, allowing vibrational frequency calculationsof larger molecules.26

The current work reports DFT analysis of the IR and Ramanspectra of a semiconductor QD capped with organic ligands.Good agreement with the available experimental data isobtained, the origin of the observed spectral shifts is elucidated,and novel predictions are made. The calculations show that QDbinding significantly influences the spectral features of thegroups that are not only chemically bound to the QD but alsointeract with the QD through space. The spectra of the directlybound groups change in both frequency and intensity. As thedistance between the QD and the functional group increases,the frequency shifts become insignificant; however, theintensities continue to change as a result of long-rangeelectrostatic interactions. Significant changes in the ligandvibrational spectra are seen even for ligands that exhibitrelatively weak interaction with the QD.

2. METHODSStructural optimization of large QD−ligand systems presentssignificant computational challenges, since subtle changes in theinitial guess and subsequent drift in nuclear positions duringoptimization can lead to saddle points and local energy minima.A high computational cost is generally required in order tolocate the global energy minimum. Saddle points obstruct thenormal-mode analysis by producing imaginary vibrationalfrequencies. Trapping in local minima can misrepresent QD−ligand structure and vibrational signals. The difficulties with thegeometry optimization originate from a relatively high flexibilityand mobility of ligands with respect to the QD surface and alarge conformational space of long ligand molecules. In order topartially remedy these problems, we used a multistepoptimization process.The initial geometry of the Cd33Se33 QD was obtained from

the bulk structure using a spherical cutoff. The geometry wasoptimized using plane-wave DFT, as implemented in theVienna Ab-initio Simulation Package (VASP).27 Then, theligands were added to the optimized QD, and geometries of thecombined QD−ligand systems were optimized. The VASPsimulations were carried out in a cubic cell periodicallyreplicated in three dimensions. In order to prevent spuriousinteractions between system’s periodic images, the cell wasconstructed to have at least 8 Å of vacuum between the replicas.The Perdew, Burke, and Ernzerhof (PBE) DFT functional28

and a converged plane-wave basis were used. The coreelectrons were treated using the projector-augmented wave(PAW) approach.29

The geometries obtained in VASP were optimized furtherusing the hybrid B3LYP functional30 with atomic Gaussianbasis sets 6-311++g(d,p)/LANL2dz for the ligand/QD systemsusing the Gaussian 09 software package.31 The core electrons ofthe heavy elements forming the QD core were treated viaeffective core potentials (ECP). ECPs include relativistic effectsthat are important in these atoms. The energy minima with

respect to the nuclear coordinates were obtained bysimultaneous relaxation of all geometric parameters using thegradient method of Pulay.32 Full convergence was achievedwith the maximum component of the force below 0.00045mdyn and the maximum displacement below 0.0018 Å. Theoptimized structures reveal that the TMPO and methylamine(MA) ligands bind to the QD by forming Cd−O and Cd−Nbonds (Figure 1).

It is well-known that vibrational frequencies obtained byquantum-chemical calculations are typically larger than theirexperimental counterparts, and thus, empirical scaling factorsare often used to better match the experimental data.33 Itshould be emphasized that the theoretical data reported belowwere not scaled, since the current work focuses on the changesin the ligand spectra induced by binding to the QD rather thanon the absolute values of the frequencies.

3. RESULTS AND DISCUSSIONThe study focuses on a small stoichiometric Cd33Se33 QD,whose geometric and electronic structure has been studiedpreviously.18,23b Cd33Se33 represents a “magic” size QD,

14 sinceit maintains a high degree of symmetry, close to spherical, andpossesses a “magic” number of atoms to create a large band gap.Upon geometry optimization the surface “self-heals”;23a i.e.,surface dangling bonds created when the structure was cut fromthe bulk are eliminated by surface reconstruction. Suchdangling bonds generate midgap states in other nonmagicstructures. Self-healing of the Cd33Se33 surface does not requireligands, while nonmagic structures generally require ligands inorder to eliminate midgap states. Therefore, the QD−ligandinteraction is weaker in magic than nonmagic size QDs. Sincesurface reconstruction should become easier with increasingQD size, the QD−ligand interaction should be stronger insmaller QDs. Thus, the QD−ligand interaction in the smallmagic size QD studied here represents an average situation.The interaction strength is larger in small nonmagic sizeclusters and smaller in large magic size QDs.

Figure 1. Optimized geometric structures of (A) Cd33Se33 withTMPO (B) Cd33Se33 with MA.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp303275v | J. Phys. Chem. C 2012, 116, 14674−1468114675

3.1. Geometric Structure of the QD−Ligand Systems.The study considers two representative ligands: TMPO andMA. Table 1 lists the optimized structural parameters for the

free ligands and for the ligands bound to the CdSe QD. Thestructural parameters are compared to the experimentalvalues34 whenever possible.TMPO binds to the Cd atom of the QD via the O atom of

the PO bond (Figure 1A). The QD−ligand interaction is ofthe dative type, involving electron donation from the occupiedd-orbitals of Cd to the vacant π* orbital of the ligand. Uponbinding, the P−O bond length increases from 0.148 nm,characteristic of the double PO bond, to 0.164 nm, typical ofthe single P−O bond. The latter value agrees very well with0.1637 nm recorded earlier.35 The changes in the PO bondlength from 0.148 to 0.164 nm and the CPC bond angles from104.97° to 108.3° and 111.8° after binding indicate that theTMPO skeleton (POC3) exists in a distorted tetrahedralconfiguration with the angles differing only by −1.2° and +2.3°from the perfectly tetrahedral 109.5°. The OPC angles of theTMPO ligand decrease by 1.8° and 5.8° after binding to theQD. The angle tightening owes to the decrease of the repulsionforces between the P−O and P−C bonds of the skeleton,caused by elongation of the P−O bond and disappearance of

the π-electron cloud. Consequently, the nearby CPC anglesgrow by 3.03° after binding to the dot. The Cd−O bond lengthis 0.218 nm, which is equal to the sum of the correspondingcovalent radii, indicating Cd−O is a covalent bond.MA binds to CdSe by forming a Cd−N bond (Figure 1B)

that is 0.242 nm long. This number is 0.02 nm larger than thesum of the covalent radii of the Cd and N atoms, suggestingcoordination rather than covalent binding between CdSe andMA. The coordination binding is of the donor−acceptor type,in which 0.2 electrons are transferred from the QD to the Natom (Table 2). The calculated Cd−N bond length is close to

the previously reported value of 0.2337 nm obtained incadmium complexes.36 In contrast to TMPO, the structuralparameters of MA are affected little by the binding, since itoccurs through the coordination bond (see Table 1).

3.2. Effect of the Ligand−QD Interaction on LigandPolarization. The distribution of charge density in a moleculeinfluences its vibrational spectrum. For instance, changes in thebond order can explain the shifts of the vibrational frequenciesof ligand groups that are bound to the QD. Ligand polarizationcan rationalize the shifts for bonds that are not directlyinteracting with the semiconductor.Table 2 presents the Mulliken charges on the TMPO and

MA atoms before and after binding to the QD. The resultsshow that the ligand−QD interaction leads to a significantredistribution of electron density. The hydrogen atoms ofTMPO carry positive charges. The H8, H11, and H14 atoms,which form a 180° dihedral angle with the oxygen atom,accommodate higher positive charges than the other hydrogenatoms, which form the dihedral angles of ±58.8°. Oxygen,

Table 1. Structural Parameters of TMPO and MA before andafter Binding to the Cd33Se33 QD Obtained with B3LYP/6-311++G(d,p)a

TMPO TMPO CdSe−TMPO

B3LYP6-311++g(d,p) expt34a

B3LYPLANL2dz/6-311++g(d,p)

r(PO) 0.148 0.148 0.164r(P−C) 0.186 0.177 0.186r(C−H) 0.111 0.111 0.109r(Cd−O) 0.218∠(C3−P−O) 113.6 112.2 111.9∠(C4−P−O) 113.6 108.3∠(C−P−C) 105.0 105.9 108.3∠(P−C−H6) 108.7 108.9∠(P−C−H7) 108.8 109.3∠(P−C−H8) 112.2 110.6∠(Cd−O−P) 134.9τ(O−P−C-H6) 58.8 56.6τ(O−P−C−H7) −58.8 −61.9τ(O−P−C−H8) 180.0 177.6

MeNH2 MeNH2 CdSe−MeNH2

B3LYP6-311++g(d,p) expt34c

B3LYPLANL2dz/6-311+

+g(d,p)

r(N−C) 0.147 0.147 0.148r(Cd−N) 0.242r(N−H3,4) 0.101 0.102 0.102r(C−H5,6) 0.110 0.110r(C−H7) 0.109 0.109∠(N−C−H5) 109.2 109.2∠(N−C−H6) 109.2 109.3∠(N−C−H7) 115.2 112.2∠(C−N−H3,4) 111.2 110.6∠(Cd−N−C) 119.0τ(Cd−N−C−H7) 179.1

aThe bond lengths and angles are in nm and degrees, respectively. Thehydrogen atom numbers are defined in Figure 1.

Table 2. Mulliken Atomic Charges of TMPO and MA beforeand after Binding to the QD Calculated Using B3LYP/6-311++G(d,p)

TMPO

atom no.a symbol before after

1 P −0.147 1.1302 O −0.265 −0.8103 C −0.243 −0.8394 C −0.243 −0.8435 C −0.242 −0.8266 H 0.100 0.2367 H 0.100 0.2388 H 0.180 0.2699 H 0.100 0.23910 H 0.100 0.26211 H 0.180 0.30212 H 0.100 0.26813 H 0.100 0.26714 H 0.180 0.305

MA

atom no.a symbol before after

1 N −0.452 −0.6792 C −0.365 −0.5453 H 0.217 0.3464 H 0.217 0.3535 H 0.113 0.2496 H 0.136 0.2467 H 0.136 0.221

aFor atom numbering, see Figure 1.



phosphorus, and carbon atoms carry negative charges. Bindingincreases both positive and negative charges on all atoms exceptfor the phosphorus, which becomes positively charged as aresult of breaking of the double PO bond. In MA, the H3 andH4 atoms attached to nitrogen are more positively charged thanthe other hydrogen atoms attached to carbons, due to thehigher electronegativity of nitrogen relative to carbon. Nitrogenand oxygen are negatively charged, and most atomic charges ofthe MA molecule increase upon binding to the CdSe QD. Theatomic charge analysis indicates that both ligands are notablypolarized by interaction with the QD.3.3. Vibrational Spectra of the QD−TMPO Complex.

The vibrational spectra of the isolated TMPO and MA ligandshave been investigated previously, both experimentally andcomputationally.22,37 A brief discussion of the vibrationalassignments is required here for comparison of the ligandsspectra before and after binding to the CdSe QDs. Figures 2−5

present the simulated vibrational spectra for the ligands beforeand after binding. The simulated spectra are compared with theexperimental data whenever appropriate. The normal modes forTMPO are distributed between two sets of fundamentalvibrationsthose arising from the methyl group and from themain molecular skeleton. Since TMPO binds to the Cd atom ofthe QD through the oxygen atom of the PO group to form aCd−O−P link, the binding should notably affect the frequencyand intensity of the PO bands in the vibrational spectra.The simulated IR spectrum for TMPO (Figure 2D) shows

good agreement with the observed spectrum (Figure 2B) in thepositions of the fundamental frequencies. The six asymmetricstretches of methyl groups form a medium intensity peak in the3125−3119 cm−1 region of the IR spectrum (Figure 2D). Thevibrations form a sharp and very strong band at the sameposition in the Raman spectrum (Figure 3B). Our calculationagrees well in both peak position and intensity with the 3124cm−1 mode calculated37b using the same level of theory(B3LYP) and the 6-311 g** basis set. The asymmetric mode ofthe methyl groups was observed in IR within the 2980−2995cm−1 range.38 The Raman analogue was seen at 2999 cm−1.37a

The methyl symmetric stretches produce a signal in the vicinityof 3041 cm−1 (Figure 2D), with the IR intensity of about 5 km/mol in agreement with the earlier calculations.37b The intensityof the corresponding Raman signal (Figure 3B) is somewhathigher (463 Å4/amu) than the earlier prediction (373 Å4/amu).37b This discrepancy can be attributed to the basis setdifference (6-311++g(d,p) vs 6-311 g**). Experimentally, thesymmetric CH stretching modes of TMPO were observed inthe 2916−2935 cm−1 spectral region.37a,38

The methyl bending modes appear at 1481 (δip), 1463 (ρ),and 1460 cm−1 (ρ). The modes are broadened in the IRspectra, while they show finer features in the Raman signal. Thecalculated frequencies are in good agreement with thosepredicted earlier at 1483, 1466, and 1462 cm−1,38c respectively.These in-plane and rocking methyl vibrations were assigned tothe bands observed at 1450 and 1408 cm−1,38 respectively. Thebands were also observed at 1437 and 1420 cm−1.37a

The wagging modes of the methyl groups appear at 959, 951,and 868 cm−1, in agreement with the earlier calculations37b

reporting 962, 953, and 868 cm−1. These wagging motions areobserved in the 860−940 cm−1 range in both IR38 andRaman37a spectra. The PO stretch was recorded at 1170cm−1 37a and 1147 cm−1 ,21 in good agreement with 1205 cm−1

seen in the calculated IR spectrum (Figure 2D). This vibrationappears in the Raman spectrum as a weak band at 1205 cm−1

(Figure 3B) with the calculated intensity of 15.2 Å4/amu. Theresult is in good agreement with the experimental signalobserved37a at 1120 cm−1.Comparing this brief vibrational assignment of TMPO with

its spectrum after binding to the CdSe QD shows differences inthe positions and intensities of the fundamental vibrations. Theshifts are attributed to the ligand−surface interaction thatpolarizes the electron density and changes the dipole momentsof the ligand modes. One can expect that vibrations involvingatoms directly bound to the QD are affected most. Themagnitude of the vibrational shifts should decay rapidly withdistance away from the QD, since TMPO is not a conjugatedmolecule, and its electron density distribution is relativelyimmobile. Rather surprisingly, shifts as large as 60 cm−1 areobserved for the C−H bonds that have no direct contact withthe QD. The shifts can be explained by changes in the chargedistribution due to the QD−ligand binding (Table 2).Interaction with the QD increases the polarization of theligand atoms: for example, the charges on the C3−H6 bond are−0.24 and 0.10 before binding and −0.84 and 0.24 afterbinding. As a result, the electrostatic interaction contributing tothe bond strength increases, and the C−H bond frequency

Figure 2. IR spectra (3500−800 cm−1) of (A) CdSe with TMPOmeasured in tetrachloroethylene, (B) TMPO measured in tetrachloro-ethylene, (C) CdSe with TMPO, and (D) TMPO. The star denotesthe PO band. Parts A and B show the experimental spectra,21 whileparts C and D present the spectra calculated in the present work.

Figure 3. Calculated Raman spectra (4000−100 cm−1) of (A) CdSewith TMPO and (B) TMPO. The star denotes the PO band.



blue-shifts. The frequency of the P−O bond connected to theCd atom (Figure 1A) red-shifts by as much as 140 cm−1. In thiscase, the changes in the atomic charges play a smaller role thanthe change in the bond order: the P−O bond switches from adouble bond in isolated TMPO to a single bond in theTMPO−QD complex. The frequency shifts can be deduced bycomparing the spectrum of TMPO attached to the QD (Figure2C) with the free TMPO spectrum (Figure 2D).All vibrations involving the hydrogen atoms of TMPO blue-

shift. The CH3 asymmetric stretch moves from 3125 to 3185cm−1. The symmetric stretch shifts less, from 3041 cm−1 in thefree ligand to a broader band at 3060 cm−1 in the IR spectrumof the TMPO−QD complex. The corresponding band in theRaman spectrum blue-shifts and splits into two strong bands at3060 and 3073 cm−1. The broadening and splitting of the CH3lines can be attributed to symmetry breaking between the threeCH3 groups due to through-space interaction with the QD(Figure 1A). The in-plane bending mode of the methyl groupsmoves from 1481 to 1508 cm−1 in IR and to 1488 cm−1 inRaman. The rocking modes shift less than the in-planedeformation, from 1463 to 1485 cm−1 and from 1460 to1470 cm−1.In the experiment,21 the sharp band of the PO vibrational

mode observed in the IR spectrum of isolated TMPO (Figure2B) is significantly broadened and strongly red-shifted from1147 to 1090 cm−1 upon binding to the CdSe QD (Figure 2A).The red-shift was confirmed by DFT calculations21 using theBP86 functional and a polarized split-valence basis set. There, a85 cm−1 shift was obtained. Our calculations show a 140 cm−1

red-shift from 1205 to 1065 cm−1. These results support thefact that binding of TMPO with CdSe through the −Cd−O−P− linkage breaks the π-bond between the phosphorus andoxygen atoms in order to create the Cd−O bond (Figure 1A).3.4. Vibrational Spectra of the QD−MA Complex. MA

presents a qualitatively different example of the QD−ligandinteraction, since the binding occurs via a coordination donor−acceptor bond, rather than a covalent bond as in the case ofTMPO.39 The analysis of the simulated vibrational spectrumfor MA bound to the CdSe QD is likely to be morecomplicated than for TMPO, due to hydrogen bonding and ahigh degree of mixing of different types of vibrations,characteristic of alkyl amines in solution.22,37 Therefore, wewill focus on the assignments of the simulated spectrum ofgaseous phase, where the interfering factors are brought to aminimum.The very weak band calculated at 3585 cm−1 (Figure 4B) can

be assigned to the asymmetric NH2 stretch, corresponding tothe 3427 cm−1 IR signal observed in the MA vapor study.37c

The calculated frequency overestimates the experimental data,as expected; note that empirical scaling factors typically used tocorrect for the systematic overestimation33 are not used here.The asymmetric NH2 stretching mode shows high Ramanintensity (58.9 Å4/amu) (Figure 5B), in agreement with thestrong band observed at 3470 cm−1 in the liquid phase.37d Thefrequency difference between vapor and liquid arises due tointermolecular hydrogen bonding. The symmetric NH2 stretchis observed in the IR and Raman spectra at 3361/3360cm−1.37d,34c The calculated frequency is 3507 cm−1.The CH stretching region in alkyl amines is usually

accompanied by overtones and combination bands of CH3deformation modes around 2900 ± 50 cm−1 in the IRspectrum, complicating the assignment.40 However, the CHstretches do appear clearly in the calculated spectrum (Figure

4B) at 3092 and 3056 cm−1 for the asymmetric motion and2961 cm−1 for the symmetric stretch. The asymmetricvibrations were observed at 2985 and 2820 cm−1, while thesymmetric mode was seen at 2961 cm−1 in the IR spectrum.37c

The NH2 bending modes are characteristic of primaryamines. The NH2 scissor mode was calculated at 1667 cm−1, ingood agreement with the IR band observed at 1623 cm−1 37c

and 1625 cm−1.34c The NH2 twisting and rocking modes withthe calculated frequencies at 1341 and 972 cm−1 are predictedto have very weak intensity in both IR (0.02/0.13 km/mol) andRaman (1.67/0.36 Å4/amu) spectra. Therefore, they are hardto observe experimentally. The twist mode was assigned to theband at 1455 cm−1,37c while no assignment for this mode couldbe made in ref 38c. The wagging mode of NH2 was calculatedat 818 cm−1 and observed at 780 cm−1 37c and 783 cm−1.34c Theband calculated at 302 cm−1 can be assigned to the NH2torsional mode that was observed at 264 cm−1.37c Three CH3bending modes are seen in the calculated spectrum at 1517(δip), 1497 (ρ), and 1460 cm−1 (δumbrella) (Figure 4B). Thesemodes were observed at 1485, 1473, and 1430 cm−1,respectively, in the IR signal.37c The CH3 wagging mode wascalculated at 1162 cm−1 and observed37c at 1130 cm−1. The C−N stretching vibration was calculated at 1055 cm−1 andobserved37c at 1044 cm−1 in the IR spectrum.

Figure 4. Calculated IR spectra (4000−200 cm−1) of (A) CdSe withMA and (B) MA.

Figure 5. Calculated Raman spectra (4000−100 cm−1) of (A) CdSewith MA and (B) MA.



MA binds to the QD through the NH2 group (Figure 1B).Therefore, one expects largest frequency shifts after binding forthe C−N, N−H stretches and NH2 bending modes. Theasymmetric NH2 stretch red-shifts by 20 cm−1 from 3585 cm−1

calculated for isolated MA to 3565 cm−1 (Figure 4A) for MAbound to the CdSe nanocrystal. Similarly, a 25 cm−1 shift to alower frequency is predicted for the symmetric NH2 stretch,from 3507 to 3482 cm−1. These shifts are relatively minorcompared to the 140 cm−1 shift in the frequency of the P−Ogroup bonded to the CdSe QD in the case of TMPO.The N−H scissor mode blue-shifts by 46 cm−1 from 1667

cm−1 for isolated MA to 1713 cm−1 in MA bound to the QD(Figure 4A). Cooper et al.22 observed a splitting in the scissormode from 1610 cm−1 to 1544 and 1635 cm−1 after bindingwith CdSe QDs. However, such splitting was not observed inthe current calculation. The experimental group attributed thesplitting to amine ligand binding to either Cd or Se surfacesites. The current work, following earlier computationalstudies,41,42 confirms that the dominant binding interactionoccurs between the nitrogen atom of the ligand and a Cd atomon the QDs surface, corresponding to the blue-shifted replica ofthe N−H scissor mode. A slight shift from 1341 to 1356 cm−1

can be noticed in the twist mode of the NH2 group. Theintensity of the mode is weak in the Raman spectrum (3.3 Å4/amu) and is essentially zero in the IR spectrum (0.4 km/mol).The most interesting difference in the calculated spectra is

the shift in the position and intensity of the NH2 waggingmode. The strong blue-shift from 818 to 1028 cm−1 can beattributed to the steric hindrance of this motion by the QD.The large frequency shift of the NH2 wagging mode provides adistinct signature for the binding of primary amines to QDs.Focusing on the methyl group, one observes a slight shift

from 1517 to 1539 cm−1 in the in-plane deformation mode.More significant shifts occur in the umbrella and rockingmodes, from 1497 to 1531 cm−1 and from 1460 to 1502 cm−1,respectively. A larger shift is seen in the methyl wagging mode,from 1162 to 1237 cm−1. The C−N stretch shifts from 1055 to1090 cm−1. This band cannot be seen in the calculated IRspectrum because its predicted intensity is low (6.6 km/mol).The C−N stretch of the ligand−QD system exhibits a mediumRaman intensity of 27.6 Å4/amu and is seen in the spectrum at1092 cm−1.Figure 6 shows selected normal modes of the ligands,

including both mode displacement and dipole-derivativevectors.

4. CONCLUSIONSThe IR and Raman spectra of TMPO and MA were simulatedby DFT calculations, before and after binding to CdSe QDs.Good agreement with the available experimental data wasobtained. The fundamental vibrations were assigned to accountfor the change in the band positions due to ligand−dotinteraction. TMPO represents a ligand that strongly interactswith the QD and forms a covalent bond. MA interacts with theQD more weakly through a coordination bond. Both ligandsbind preferentially to Cd atoms of the QD through the POand NH2 groups for TMPO and MA, respectively.The main observations for the TMPO spectra include a

strong red-shift in the PO stretching mode frequency due toits transformation into a single bond and splitting of severalfundamental modes of the methyl groups due to reduction ofthe C3v TMPO symmetry upon binding to the QD. The POred-shift is most easily observed in the IR spectrum, while the

CH3 splitting can be detected in the Raman spectrum. Thebinding of MA to the QD can be most easily detected by astrong blue-shift of the NH2 wagging mode frequency, arisingdue to steric hindrance of this motion by the QD.The reported results establish the key QD−ligand interaction

mechanisms that are responsible for the changes in the ligandvibrational spectra, demonstrating that vibrational spectroscopyis a powerful tool for examining the properties of QD surfacesand QD−ligand interactions that are of interest for bothfundamental research and emerging technological applications.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] Address§University of Nizwa, College of Arts and Sciences, Post Code616, P.O. Box 33, Sultanate of Oman.Author Contributions⊥Taken in part from the PhD Thesis of Mr. Ahmed Abulelelato be submitted to Al Azhar University, Faculty of Science, NasrCity, Cairo, Egypt.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to Amanda Neukirch for comments onthe manuscript. The research was supported in part by the NSFgrant CHE-1050405.

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Figure 6. Selected normal modes of the ligands. The normal modedisplacement vectors are shown in blue, and the dipole-derivative unitvectors are shown in orange.



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dft simulation and vibrational analysis of the ir and

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