Luminescent GaN semiconductor based on surface modification withlanthanide complexes through an ionic liquid bridge{

Qiu-Ping Li and Bing Yan*

Received 30th March 2012, Accepted 16th September 2012

DOI: 10.1039/c2ra20582c

A novel GaN-based luminescent hybrid material has been

prepared by covalently functionalizing the GaN matrices with

an ionic liquid (IL), and then exchanging the anion of the IL with

lanthanide complexes. 1-Methyl-3-[3-(trimethoxysilyl)propyl]imi-

dazolium chloride, a kind of multi-functional room temperature

IL, is used to silanize the hydroxylated surfaces of the GaN

sample. After that, an anion exchange measurement is performed

to introduce the tetrakis b-diketonate europium(III) complex

anion to the GaN matrices. The FTIR spectra, UV-vis diffuse

reflection absorption spectra, scanning electron micrograph,

XRD patterns and photoluminescent properties (luminescence,

lifetime and quantum efficiency) for the resulting material were

studied in detail. The results suggest that our method is an

effective way for constructing novel functional GaN based hybrid

materials.

During the last few years, gallium nitride (GaN) has attracted

increasing interest as a substrate material for various applications

because of its outstanding chemical stability under tough conditions,

physical hardness, well electron mobility and good optical properties.

The mass production of bulk GaN crystals presents opportunities for

designing GaN-based functional materials,1 such as light-emitting

diodes, chemical or biological sensors, power electronics and

photovoltaic devices.2 Many works have been carried out to

construct GaN-based luminescent materials by doping lanthanide

elements to GaN matrices or attaching a luminescent phosphor to

the surface of it.3 Before the attachment of the luminescent

phosphors, the GaN must be pretreated to provide an oxidized

and hydrogen-terminated surface for the further immobilization of

luminescent molecules.4 It has been proven that the silanization of

hydroxylated surfaces is an effective way to construct silane-based

multifunctional materials, which have been investigated in numerous

works during the last decades.5 The successful covalent functiona-

lization of GaN surfaces with organosilanes has been demonstrated

and confirmed by several reported literatures.6 Recent examples of

this sort of materials are the GaN-based microsensors which are

constructed by covalently functionalize the GaN surface with

luminescent Ru2+ complexes or long-lived luminescent indicator

dyes.7

Recently, room-temperature ionic liquids (IL) are gathering lots

of interest as environmentally benign solvents for organic

synthesis and separation,8 and some recent studies focus on their

use in material science.9 The most common ILs include

alkylammonium salts, alkylphosphonium salts, imidazolium salts

as well as N-alkylpyridinium salts. Among which the imidazolium

salts are widely applied in constructing functional materials and

have found a variety of applications owing to their distinctive

properties.10 One of the best-studied ways of using it is to

immobilize the imidazolium salts on various supporting matrices,

which have been published many times in recent years, such as

silica-supported imidazolium salt derived catalyst, and silica-

based imidazolium stationary phases used in liquid chromato-

graphy.11 Since the tetrakis b-diketonate lanthanide(III) com-

plexes can be electrostatically coupled to an IL, it presents

opportunities for constructing imidazolium salt-derived lumines-

cent materials, which has been confirmed by several previous

literature.12 But to our best knowledge, there are no previous

reports about functionalizing GaN with an IL.

In this Communication, we report on the covalent functionaliza-

tion of GaN surfaces with an imidazolium salt IL, which bears an

organosilane group and can silanize the hydroxylated surfaces of

GaN and then can adsorb the tetrakis b-diketonate europium

complex anion through the electrostatically driven anion exchange

reaction. The aim of the work described herein is to explore a way of

constructing GaN-based luminescent materials using the IL and

research their photophysical properties.

Fig. 1 shows the detailed synthetic pathways to obtain the

luminescent materials. GaN powder is provided by Alfa Aesar, while

all the other reagents and solvents are obtained from Aladdin and

used without further purification. In a typical experiment, the GaN

powder is treated with piranha solution (H2SO4/H2O2 (3 : 1 v/v)) for

30 min, and then washed with deionized water and dried under

vacuum condition prior to silanization.7,13 The oxidized sample is

then silalized by ultrosonicating it in the as-prepared 1-methyl-3-[3-

(trimethoxysilyl)propyl]imidazolium chloride (room temperature IL,

marked as TMOSIM+Cl2) toluene solution under 50 uC. After one

hour, a uniformly IL-coated material is obtained, and the unbound

IL is removed by washing with enough ethanol. At the same time the

tetrakis b-diketonate europium(III) complex NEt4Eu(TTA)4 (TTA =

thenoyltrifluoroacetone) was prepared by following a modified

Department of Chemistry, Tongji University; State Key Lab of WaterPollution and Resource Reuse (Tongji University), Siping Road 1239,Shanghai 200092, China. E-mail: byan@tongji.edu.cn;Fax: +86-21-65981097; Tel: +86-21-65984663{ Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra20582c

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literature procedure.14 In order to anchor the luminescent phosphor

[Eu(TTA)4]2 onto the surface of GaN sample, we perform an anion

exchange reaction, typically, NEt4Eu(TTA)4 is added to a dispersion

of IL-bearing GaN powder in ethanol and stirred for 24 h at room

temperature (RT). Finally, the GaN powder is rinsed with sufficient

ethanol to remove any trace of NEt4Eu(TTA)4 and dried under

vacuum conditions. The resulting material is denoted as GaN-IM+-

[Eu(TTA)4]2.

The success of surface functionalization for GaN has been proven

by the compare of IR spectra between the neat GaN and the resulted

material GaN-IM+-[Eu(TTA)4]2 presented in Fig. 2. As shown in

the IR spectrum of GaN-IM+-[Eu(TTA)4]2, the weak broad band

centered around 3110 cm21 can be assigned to the vibration of (LC–

H). The IR band for u(CLO) vibrations of the mono-deprotonated

ligand TTA appears in a lower frequency region (centered around

1625 cm21), which can be ascribed to the complexation of the Eu3+

ion with the oxygen atom of the CLO. The characteristic absorbance

of the imidazole ring bend is found at 1577 cm21.15 Besides, the

absorption bands centered at 1309 cm21 and 1137 cm21 can be

assigned to the symmetric and asymmetric stretching vibrations of

the –CF3 group. In addition, the absorption peaks appearing at

621, 580 and 561 cm21 are ascribed to the GaN matrices, it can be

observed in both the spectra. Compare the IR spectrum of GaN-

IM+-[Eu(TTA)4]2 to the one of GaN, all the newly arisen absorption

peaks which we have discussed above suggest that the europium

complexes have been chemically immobilized on the surface of GaN.

The X-ray photoelectron spectroscopy (XPS) analysis of

samples before and after anion exchange reaction is provided as

a complement to FTIR. As seen in the left part of Fig. 3, the

emerging peak of F 1s appearing at 692 eV in GaN-IM+-

[Eu(TTA)4]2 is positive evidence for the success of anion

exchange. According to the previous report, the O 1s core-level

XPS spectra of IL modified GaN could be attributed to the O–H,

O–Ga and O–Si constituent.6 Thus, for the materials after anion

exchange, the new component (around 536.3 eV) could be

attributed to the introduction of coordinated oxygen within the

resonating structures of chelated b-diketone ligands.

The thermogravimetric (TG) and the corresponding derivative

weight loss (DTG) analyses have been performed for the IL-

modified GaN and GaN-IM+-[Eu(TTA)4]2. As shown in Fig. 4A,

the IL-modified GaN shows a typical two-step weight loss

approach over 225 uC according to the DTG curve, which is

coincidental with the weight loss phenomenon observed in

previously reported analogous IL-modified materials.16

Similarly, based on the DTG curve presented in Fig. 4B, we have

found that the GaN-IM+-[Eu(TTA)4]2 shows an obvious three-

step weight loss procedure beyond 225 uC. Apparently, the second

procedure of weight loss between 300 and 450 uC could be

associated with the decomposition of europium tetrakis(b-dike-

tonate) attaching onto the GaN matrices through electrostatic

interactions,17 which also can be seen as complementary evidence

for the success of anion exchange. In addition, the residual weight

of GaN-IM+Cl2 and GaN-IM+-[Eu(TTA)4]2 is essentially

contributed by the same composition. After normalization to

the residual weight of GaN-IM+-[Eu(TTA)4]2, reasonable loading

percentages for the IL (5.40%) and europium complex (5.17%) are

deduced. The loading percentage deviation (0.33%) of IL between

IL-modified GaN and GaN-IM+-[Eu(TTA)4]2 is acceptable.

Fig. 2 Infrared spectra of pure GaN and GaN-IM+-[Eu(TTA)4]2.

Fig. 3 F 1s and O 1s core-level XPS spectra of (1) the material before anion

exchange (IL-modified GaN) and (2) the material after anion exchange

(GaN-IM+-[Eu(TTA)4]2).

Fig. 1 Synthetic scheme for the preparation of lanthanide complex-

functionalized GaN.

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The X-ray diffraction (XRD) patterns (Fig. S1, ESI{) of the pure

GaN and GaN-IM+-[Eu(TTA)4]2 are determined at room tempera-

ture within the 2h range of 10–70u. Both of the spectra show similar

sharp peaks that originate from the crystal-line-natured GaN, which

means the crystal structure of GaN matrices has been well preserved

after the surface functionalization although the peak intensities in the

XRD pattern of GaN-IM+-[Eu(TTA)4]2 show a slight decrease.

According to the XRD patterns, both of the samples display a

hexagonal structure which is in good agreement with the JCPDS file

(JCPDS 65-3410 Hexagonal-type, space group P63mc). In addition,

the scanning electron micrograph (SEM, Fig. S2, ESI{) for GaN-

IM+-[Eu(TTA)4]2 shows a homogeneous and regular microstruc-

ture. Based on the SEM and XRD patterns, we conclude that the

immobilization of IL and anion exchange have little influence on the

microstructure of the GaN matrices. The ultraviolet–visible diffuse

reflection absorption spectra of GaN-IM+-[Eu(TTA)4]2 is performed

on powdered samples and presented in Fig. S3, ESI.{ It can be

observed from the figure that a broad absorption band is located in

the range 200–600 nm. Moreover, the spectra of GaN-IM+-

[Eu(TTA)4]2 peaks at about 360 nm, which is coincident with the

dominant peaks of the excitation spectra for GaN-IM+-[Eu(TTA)4]2

shown in Fig. 5. In addition, we can observe an obvious inverse peak

at about 614 nm, the characteristic transition of a europium ion

under excitation during the measurement.

The excitation and emission spectra of the obtained GaN-IM+-

[Eu(TTA)4]2 is shown in Fig. 5. The excitation spectrum is obtained

by detecting the characteristic emission of a europium(III) ion at 614

nm and dominated by a broad band centered at about 360 nm in the

ultraviolet region. The broad excitation band suggests that the

combination of TTA and functionalized GaN matrices can sensitize

the transition of the europium ion effectively. The emission spectra of

GaN-IM+-[Eu(TTA)4]2 is obtained by using the most appropriate

wavelength (360 nm) as the excitation wavelength based on its

excitation spectra. As shown in the right side of Fig. 5, the emission

lines are assigned to the transitions 5D0 A 7FJ (J = 0–4) located at

578, 589, 614, 651, and 699 nm for europium ion. The emission

spectrum is dominated by the very intense 5D0 A 7F2 transition at

614 nm, suggesting that an effective energy transfer takes place from

the matrices to the europium ion. Besides, since the 5D0 A 7F1

transition of the europium ion is a parity-allowed magnetic dipole

transition and relatively independent from its chemical environment,

while the 5D0 A 7F2 transition is a typical electric dipole transition

and highly sensitive to its environment, we can use the intensity ratios

I(5D0 A 7F2)/I(5D0 A 7F1) as an indicator for the local environment

of europium ion. Here, the intensity ratio of the red/orange intensities

is approximately 16.1 (Table S1, ESI{), indicating that the

europium(III) ions are located at an asymmetric environment. In

order to further investigate the photoluminescence property, the

decay curve of GaN-IM+-[Eu(TTA)4]2 is detected and fitted (Fig. 6)

into a single exponential function in the form ln(St/S0) = 2k1t = 2t/

t. From which, the lifetime value is calculated as 316 ms.

Furthermore, based on the emission spectra and the lifetime (t) of

GaN-IM+-[Eu(TTA)4]2, the emission quantum efficiency (g) of the

5D0 excited state of europium(III) ion is determined to be 26.3%

according to ref. 18 (details for calculation process of luminescence

quantum efficiency can be seen in ESI{).

In summary, we have successfully functionalized the GaN

surface with europium complexes by immobilizing a novel IL to it

and then performing an anion exchange. The tetrakis b-diketonate

europium(III) complex anion has been chemically bonded to the

surface of GaN through an imidazolium salt molecular bridge

under the drive of an electrostatic interaction, resulting in a novel

GaN-based luminescent material. The physical properties, espe-

cially the photophysical properties, have been investigated in

Fig. 4 TG and DTG curves for IL-modified GaN (A) and GaN-IM+-

[Eu(TTA)4]2 (B).

Fig. 5 Excitation and emission spectra of GaN-IM+-[Eu(TTA)4]2.

Fig. 6 Luminescent decay curves of GaN-IM+-[Eu(TTA)4]2. (Excitation

wavelength = 360 nm; emission wavelength = 614 nm; circles: experimental

data; solid line: fitted according to I = I0 + Aexp[2(t 2 t0)/t.]

10842 | RSC Adv., 2012, 2, 10840–10843 This journal is � The Royal Society of Chemistry 2012

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detail. The photoluminescent properties reveal that GaN is a

favorable matrix for constructing luminescent rare earth hybrid

materials. Moreover, this achievement provides a new approach

to design novel functional GaN materials.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (20971100, 91122003), Program for New

Century Excellent Talents in University (NCET 2008-08-0398).

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