Electronic structure and optical properties of resin

Electronic structure and optical properties of resin

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 618–622 Contents lists available at SciVerse ScienceDirect Spectrochi...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 618–622

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Short Communication

Electronic structure and optical properties of resin Zhi-Fan Rao ⇑, Rong-Feng Zhou Analysis and Testing Center of Yunnan, Kunming University of Science and Technology, Kunming 650093, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" The electronic structure of resin is

The crystal structure of resin. The color of C, O and H is white, red and gray, respectively.

gotten by calculation firstly. " The band gap of resin is 1.7 eV. " The optical properties of resin are calculated by first principles.

a r t i c l e

i n f o

Article history: Received 5 December 2012 Received in revised form 3 January 2013 Accepted 6 January 2013 Available online 12 January 2013 Keywords: Organics Amber Density of states Absorption First principles calculation

a b s t r a c t We used the density of functional theory (DFT) to study the electronic structure and density of states of resin by ab initio calculation. The results show the band gap of resin is 1.7 eV. The covalent bond is combined C/O atoms with H atoms. The O 2p orbital is the biggest effect near the Fermi level. The results of optical properties show the reflectivity is low, and the refractive index is 1.7 in visible light range. The highest absorption coefficient peak is in 490 nm and the value is 75,000. Ó 2013 Elsevier B.V. All rights reserved.

Introduction More broadly, the term ‘‘resin’’ also encompasses a great many synthetic substances of similar mechanical properties [1]. However, resins consist primarily of secondary metabolites or compounds that apparently play no role in the primary physiology of a plant. The resin produced by most plants is a viscous liquid, composed mainly of volatile fluid terpenes, with lesser components of dissolved non-volatile solids which make resin thick and sticky [2]. ⇑ Corresponding author. Tel.: +86 0871 5183273; fax: +86 0871 5111617. E-mail address: [email protected] (Z.-F. Rao). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.01.007

The most common terpenes in resin are the bicyclic terpenes alpha-pinene, beta-pinene, delta-3 carene and sabinene, the monocyclic terpenes limonene and terpinolene, and smaller amounts of the tricyclic sesquiterpenes, longifolene, caryophyllene and deltacadinene [2–4]. Some resins also contain a high proportion of resin acids. The individual components of resin can be separated by fractional distillation [5]. Solidified resin from which the volatile terpene components have been removed by distillation is known as rosin. Typical rosin is a transparent or translucent mass, with a vitreous fracture and a faintly yellow or brown color, non-odorous or having only a slight turpentine odor and taste [6,3,7]. The hard transparent resins, such as the copals, dammars, mastic and

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sandarac, are principally used for varnishes and adhesives, while the softer odoriferous oleo-resins (frankincense, elemi, turpentine, copaiba) are more largely used for therapeutic purposes and incense. Brody and his coworkers have studied the difference between amber and copal resin using FT-Raman spectroscopy, and they found the main difference was in the wavenumber regions 1771– 1543 cm1 and 1510–1400 cm1 [5]. Barletta and his coworker have researched the amber by high resolution UHV–AFM surface analysis [8]. Raman spectroscopy is a molecular spectroscopic technique that during recent years often has been used for the non-destructive analysis of historical and artistic materials [6]. Edwards et al. [3,9,10] addressed the provision of preliminary spectroscopic assignments and concentrated principally on the spectroscopic differentiation between real and imitation ambers, e.g. plastics [11]. Moreno et al. [12] report values for this ratio and suggest spectral differences exist between ambers from different geographical locations [7]. Nevertheless, the optical properties of resins are less in the previous literature. Isago et al. [13] has studied the amber phthalocyanines using spectral technology, and the optical properties were studied by absorption and magnetic circular dichroism spectroscopy. In this paper, we have built the structure of resin and calculated the electronic structure and optical properties. This paper supplied a quickly method to get the properties of a number natural compound by calculation.

Methods and details Resin is the fossilized chemical reactions including evaporation of volatile components, terpenes, alcohols, esters, etc. Thus, the

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colors range from almost pure orange through dark yellow. The differences in color are partly associated with the presence of minor inclusions filled with air, water or sap. A complex general formula of resin is C512H824O412 [3]. However, the main composition of most resin is mixture and it has a complex composition. The custom structure is showed in Fig. 1. First principles calculations based on the density of functional theory were applied to study the stability and electronic properties of resin by CASTEP code [14]. The interactions between valence electrons and ionic core were represented by ultra-soft pseudopotentials. The exchange correlation energy is calculated both by LDA and GGA in terms of CA–PZ and PBE schemes, respectively [15]. Valence electrons configuration considered in this study were including, O 2s2 2p4, C 2s2 2p2, H 1s1. The average force acting on ions was finally reduced to 1.0  106 eV/atom. Monkhorst–Pack scheme was used for k point sampling in the first irreducible Brillouin zone as 10  10  10. The maximum energy cutoff value is 400.0 eV for plane wave expansion [16].

Results and discussions The calculated band structure (BS) of resin is displayed in Fig. 2a, along the symmetry lines of the first Brillouin zone (BZ). The high symmetry points in BZ are Z (0, 0, 0.5), G (0, 0, 0), Y (0, 0.5, 0), A (0.5, 0.5, 0), B (0.5, 0, 0), D (0.5, 0, 0.5) and E (0.5, 0.5, 0.5), respectively. It can be seen that the energy gap is 1.7 eV. The property may like a semiconductor. By analyzing the reason, we think the bond would be weekend in the solidification and some fluid terpenes include the double [email protected] bond. From this point of view, the valence and conduction bands overlap

Fig. 1. (a) Some samples of resin, and its color; (b) the crystal structure of resin and the chemical formula is C512H824O412.

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considerably at the Fermi level is in good agreement with experiments [17]. In addition, the band gap is medium and the most physics properties are similar with the semiconductor, like optical properties, etc. Although it is a kind of molecular crystals, the low melting point as organic matter can be applied to many fields [10]. The calculated density of states (DOSs) of resin is illustrated in Fig. 2b, including total density of states (TDOSs) and project density of states (PDOSs) of atoms in the structure. From Fig. 2b, the 2s band of O locates at the lowest energy level, approximately 23 to 15 eV of three sharp peaks in the compound, and the O 2p can be neglected in this part. The lowest peak of C 2s atom is not similar with the O atom but it is tight binding with C 2p orbits. The reason is the tight bond with H atoms. The position of next peak of C 2p is similar with the O 2s [18]. It indicates the obvious hybridization of atomic orbits between O 2s and C 2p in very low energy level, and this is the characterization of CAO bond. In the range of 10 eV to 0 eV, both O 2p and C 2p are bonding with H atom, and the H 1s and 2p (O and C atom) are hybridized [19].

Above the Fermi Level, Two sharp peaks can be seen in density of states for C and O atoms. On the other hand, the upper covalent band is attributed to 2p band of O especially near the Fermi level [5,20]. The larger electronic dispersion of this structure is decided by the high level of C and O atoms. The optical properties of resin are very important in its applications. Fig. 3 shows the optical properties of resin by first principles calculation, like reflectivity, refractive index, absorption coefficient and dielectric function. The optical properties of resin can be described by means of the complex dielectric function e = e1 + ie2; the imaginary part of dielectric constant (e2) can be calculated theoretically based on density functional theory (see Eq. (1)), and then by applying the K–K relation we can evaluate the real part (e1) at the same time [21]. The peaks appearing in the e2 part of dielectric function are directly related to different intra-or inter band transitions in the first irreducible Brillouin zone. The absorption spectrum is calculated using Eqs. (1) and (2) [20].

(a)

(b)

8

H 1s

C PDOS

H PDOS

EF 4

0 24 20 16 12 8 4 0 12

-20

-10

10

20

C 2s C 2p

-20

O PDOS

0

-10

0

10

20

O 2s O 2p

8 4 32 0 -20

-10

0

10

Total

TDOS

24

20

16 8 0 -25

-20

-15

-10

-5

0

5

10

15

20

Energy ( eV ) Fig. 2. (a) Band structure of resin, (b) the total density of states (TDOSs) and project density of states of atoms (PDOSs) of resin. The band gap is 1.7 eV and the dash line is Fermi level (EF).

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(a)

(b) 4

0.4

3

Reflectivity

Refractive Index

Reflectivity

0.3

0.2

n k 2

1

0.1

0

500

1000

1500

2000

2500

500

3000

1000

Energy ( nm )

1500

2000

2500

3000

Energy ( nm )

(d) 12

(c) 80000 Absorption

10

Dielectric Function

Absorption

8

40000

6 4

Re Im

2 0 -2

0 500

1000

1500

2000

2500

3000

500

1000

1500

2000

2500

3000

Energy ( nm )

Energy ( nm )

Fig. 3. The optical properties of resin. (a) Reflectivity, (b) refractive index, (c) absorption coefficient, and (d) dielectric functions.

e2 ðxÞ ¼

Ve2 2phm2 x2

Z

3

d k

X 0 jhknjpjkn ij2 f ðknÞ  ð1 nn0

0

 f ðkn ÞÞdðEkn  Ekn0  hxÞ

IðxÞ ¼ 2x

½e21 ðxÞ þ e22 ðxÞ1=2  e1 ðxÞ 2

ð1Þ !1=2 ð2Þ

As we all know, the interaction of a photon with the electrons in the semiconductor system can lead to the transitions between occupied and unoccupied states. The spectra resulting from these excitations can be described as the joint densities of states between valence band and conduction band. From Fig. 3, it can be clearly seen the photon wavelength is 300–3000 nm, which is the range of the visible light and near infrared light (NIR). Fig. 3a shows the reflectivity of the resin. There is a peak near the 480 nm and a valley is near the 780 nm. So a lot of light could get inside of the resin [2]. In addition, it describes the linear response of the crystal system to electromagnetic radiation, which is mainly related to the electronic structures. The imaginary part e2 of dielectric function derives from the appropriate momentum matrix element between the occupied and the unoccupied wave functions within the selection rules over the Brillouinzone [22]. Fig. 3b shows the refractive index (n) of the resin. It can be seen the refractive index is about 1.8 (for the green light). According to Lambert–Beer’s law that the absorption coefficient I(e) is proportional to the absorbance A(e), the calculations between the calculated I(e) and the experimentally measured A(e) is in good confirmed I(e) can be performed to compare with the experimentally measured A(e). The curve is smooth. There is only

one peak of absorption coefficient in Ultraviolet light range [5]. The highest wavelength position of the peak is 490 nm and the absorption coefficient is about 75,000. After the highest peak of absorption, the absorption coefficient is declined all the high wavelength range [23] All of these would help the resin to own their sheen. Distinguishingly, the imaginary part is level off to zero in visible light range, which is different with other mineral materials. Conclusions The natural structure of resin is studied by the first principles calculation based on density of functional theory. The results show that the band gap of resin is 1.7 eV. The covalent bond is combined C/O atoms with H atoms. The O 2p orbital is the biggest effect near the Fermi level. The reflectivity is low, and the refractive index is 1.7 of green light range. There is only one peak in the absorption. The highest absorption coefficient peak in 490 nm is about 75,000. It can be see the low absorption coefficient in all of the visible light area that the resin would have a pretty sheen. Acknowledgements This work was supported by the National Natural Science Foundation (No. 51261011) and Science and Technology Program of Yunnan (2010DH025). References [1] B. Marshall, S.B. Levy, Nature 286 (1980) 650–654. [2] A. Yoshihara, T. Maeda, Y. Imai, Vib. Spectrosc. 50 (2009) 250–256.

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