Synthesis and White Light Emission of Rare Earth-DopedHfO2 Nanotubes

Lixin Liu, YuanWang, Yurong Su, Ziwei Ma, Yizhu Xie, Haiting Zhao, Changcheng Chen, Zhenxing Zhang,and Erqing Xiew

Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University,Lanzhou 730000, China

Luminescent nanotubes based on rare earth (RE) (Eu, Tb)-doped hafnia (HfO2) were prepared by radio frequency (RF)sputtering with electrospun polyvinylpyrolidone (PVP) nanofi-bers as templates. The nanotubes annealed at 5001C have auniform structure with desired tubular morphologies. Variouscolors of photoluminescence (PL) could be easily obtained bytuning the concentrations of the RE ions in the HfO2 host. TheEu- and Tb-codoped HfO2 (HfO2:Eu&Tb) nanotubes exhibitedwhite light emissions under a 325 nm excitation. Good Com-mission International de I’Eclairage (CIE) coordinates (0.333,0.323) and color temperature (Tc) (5465 K) were achieved in theHfO2:Eu&Tb nanotubes. Experimental evidence showed thatthe presence of blue emission originates from the defect states inHfO2 nanotubes, the green and the red emissions can be attrib-uted to the inner 4f shell transitions of corresponding Tb31 ionsand Eu

31ions in HfO2 shells, respectively. Compared with RE-

doped HfO2 films, the nanotube samples showed intense whitelight emissions, revealing that the HfO2:Eu&Tb nanotubes arean efficient white light-emission material.

I. Introduction

DURING the past few years, transition metal oxide HfO2 hasattracted great attention in electronics and optoelectronics

due to its high thermal, chemical, and mechanical stability, aswell as high refractive index and dielectric constant. However,the nature of defects in HfO2 influences the carrier mobility andtrapping, leakage current, and other important issues, thushindering its dielectric performance in microelectronics. Never-theless, this material may be attractive for luminescence appli-cations in the visible and UV spectral range where the emissionof impurities from intrinsic defects can occur within the opticaltransparency window. In addition, rare earth (RE) ions possessabundant energy levels, long lifetime excited states, and goodchemical stability, and can be easily excited. Therefore, the in-corporation of RE ions into HfO2 has attracted a strong interestfor fundamental studies and technical applications. Recently, anumber of papers concerning the photoluminescence (PL) of REions in HfO2 have been published.1–4 Moreover, low-dimen-sional tubular architectures have attracted tremendous interestbecause of the unique chemical and physical properties associ-ated with the high aspect ratio nanometer-sized structures.5–7 Ithas been demonstrated that doping luminescent RE ions intonanohosts is a promising approach in developing efficient andstable nanophosphors.8–10 However, until now, there have beenfew reports on the luminescent properties of low-dimensionalHfO2:RE materials, which will have potential applications in

light-emission nanodevices, such as light-emitting diodes andflat panel displays. Additionally, various colors of luminescence,including white light emission, can be obtained by tuning theconcentration of the RE ions in the host material. Consequently,it is necessary to methodically probe the PL properties of suchnanostructures and to investigate the possible energy-transfermechanism for the light emission.

A number of processes have been demonstrated for the fab-rication of nanotubes of metal oxides with diameters rangingfrom nanometers to micrometers.11–13 HfO2 nanotubes havebeen prepared by atomic layer deposition into anodic aluminumoxide nanopore arrays and electrochemical anodization.7,14

Electrospinning and sputtering together provide a simple syn-thetic technique for preparing nanotubes. Electrospinning hasprovided a simple approach to fabricate exceptionally uniformnanofibers with long length, very thin diameter, and diversifiedcomposition.15–17 Then, a metal oxide nanocoating can be easilydeposited onto the nanofibers using a sputtering process.18 Thistechnique allows a nearly homogeneous coating of three-dimen-sional materials.19,20 Finally, hollow metal oxide nanotubes canbe achieved by removing the polymer fiber templates.

In this work, we suggest the approach to efficiently fabricateRE-doped HfO2 nanotubes by using a sputtering technique withelectrospun polyvinylpyrolidone (PVP) nanofibers as templates.The nanotubes fabricated in our experiment have a uniformstructure with desired tubular morphologies. Efficient whitelight emission with color coordinates (0.333, 0.323) can be ob-tained via the fine tuning of the concentration of the RE31 ionsin the host material. The structural and optical properties of theHfO2:Eu

31&Tb31 nanotubes were studied in detail.

II. Experimental Details

HfO2:RE nanotubes were fabricated by sputtering RE-dopedHfO2 shells onto electrospun PVP nanofiber templates. Firstly,PVP nanofibers were prepared by the electrospinning tech-nique (Fig. 1(a)).20 0.2 g PVP (Sigma-Aldrich, St. Louis, MO,Mw �1 300000) was dissolved in 4 mL ethanol (� 99.7%) fol-lowed by vigorous magnetic stirring for 2 h. Then, the solutionwas pumped through a thin stainless needle with an inner di-ameter of about 0.5 mm. This needle simultaneously serves as anelectrode, to which a high electric field of 100 kV/m was applied,and the distance between the needle and the counter collectorwas maintained at 18 cm during the electrospinning process. Theelectrospun fibers were collected on n-type silicon (100 O � cm)chips that were located on the electric collector.

Then, the as-spun PVP nanofibers were put into a radio fre-quency (RF) reactive magnetron sputtering chamber for pre-paring HfO2:RE shells (Fig. 1(b)). The sputtering parameters ofHfO2:RE shells are summarized as follows: hafnium target(99.95%) of 4 in. diameter; Eu2O3 and Tb flakes were placedon the target to prepare HfO2:Eu, HfO2:Tb, and HfO2:Eu&Tbshells; pure argon (99.99%) of 21.2 sccm; pure oxygen (99.99%)of 10 sccm; RF power of 200 W; deposition pressure of 0.2 Pa.During the sputtering, the quantity of Tb metal is fixed while

S. Bhandarkar—contributing editor

This work was supported by the NSAF Joint Funds of the National Natural ScienceFoundation of China (Grant No. 10776010).

wAuthor to whom correspondence should be addressed. e-mail: xieeq@lzu.edu.cn

Manuscript No. 28348. Received July 18, 2010; approved December 10, 2010.

Journal

J. Am. Ceram. Soc., 94 [7] 2141–2145 (2011)

DOI: 10.1111/j.1551-2916.2010.04375.x

r 2011 The American Ceramic Society

2141

only changing the area of Eu2O3 to sputter different molar ratiosamples. Ultimately, the samples were annealed at 5001C underO2 ambient for 2 h to remove the PVP cores, and the HfO2:REnanotubes were obtained (Fig. 1(c)). For comparison, undopedand RE-doped HfO2 films were also prepared at the same sput-tering parameters.

The RE ion concentrations of the samples were analyzedby inductively coupled plasma optical emission spectroscopy(ICP-OES, Varian, Palo Alto, CA). The morphologies of theHfO2 nanotubes were characterized by field-emission scanningelectron microscope (Hitachi S-4800, Hitachi Ltd., Marunouchi-1, Chiyoda, Tokyo, Japan) and transmission electron micro-scope (TEM, JEM-2010, JEOL Ltd., Akishima, Tokyo, Japan).Grazing-angle X-ray diffraction (XRD, Philips, Amsterdam, theNetherlands, X’Pert Pro, CuKa1, 1.54056 A, 11) and Ramanspectroscopy (micro-Raman, Jobin-Yvon, Minami-ku, Kyoto,Japan, J. Y. HR 800, 325 nm) were used to characterize thecrystal structure of the samples. PL spectra of the HfO2 nano-tubes were recorded on a spectrophotometer (SHI-MADZU,Shimadzu Corp., Nakagyo-ku, Kyoto, Japan, RF-540) using a15 mW He–Cd laser with a wavelength of 325 nm as the exci-tation source.

III. Results and Discussion

(1) Fabrication of HfO2 Nanotubes

Figure 2 shows the SEM and TEM images of the HfO2 nano-tubes after the removal of PVP templates by annealing. TheSEM image shown in Fig. 2 indicates that the nanotubesannealed at 5001C have a uniform and smooth structure witha high aspect ratio. The cross-section SEM image of a nanotubein the inset of Fig. 2 shows that the nanotube has an intact tu-bular morphology with an outer diameter of about 200–250 nm,while the tube thickness is not homogeneous because the sput-tering only focuses on the upper side of the sample, and thebackside of the fibers is prepared by detouring of the high-energy sputtering ions. The bottom right inset of Fig. 2 displaysa typical TEM image of an annealed HfO2 nanotube. It can beseen that the nanotube has a tubular shell with an asymmetricwall thickness of 33 and 40 nm for each side. This is consistentwith the results of SEM, and the nanotubes wall thickness isabout in the range of 25–55 nm.

The crystal structure of HfO2 was determined by XRDand Raman spectroscopy. The XRD pattern of the HfO2 nano-tubes is shown in Fig. 3(a). The pattern is in good agreementwith the reference pattern of monoclinic HfO2 (JCPDS No. 78-0050), and there are no phases of Tb and Eu ions. The peaks

have been indexed in the figure, and the sample shows a pref-erential (�111) orientation. The crystal size of the nanocrystal-lite, which was calculated from the two stronger reflections(�111) and (111) using the Scherrer equation, is about 7.9 nm.The existence of monoclinic HfO2 in the nanotubes is also con-firmed by micro-Raman spectroscopy. In Fig. 3(b), the Ramanspectrum shows that the Raman peaks located at around 236,252, 327, 379, 396, 499, 551, 582, 641, 671, and 770 cm�1 (thepeak of 520 cm�1 is the signal of Si substrate) can be assigned tothe Ag and Bg modes of monoclinic HfO2.

21–23 The spectrumobtained from the monoclinic HfO2 has a low signal-to-noiseratio, and the low quality of the scattering peaks is evident fromthe Raman spectrum. This might be due to the reduced phonon

Fig. 1. Schematic illustration of the synthesis of HfO2:RE nanotubes.(a) PVP nanofibers are prepared by electrospinning. (b) Formation ofHfO2:RE shells on the as-spun PVP nanofibers by RF-reactive sputter-ing. (c) Removal of the PVP cores through annealing to yield HfO2:REnanotubes. PVP, polyvinylpyrolidone; RE, rare earth; RF, radiofrequency.

Fig. 2. Scanning electron microscopic (SEM) image of the HfO2 nano-tubes annealed at 5001C. The top right inset is a cross-section SEM im-age of a nanotube, and the bottom right inset is a typical transitionelectron microscopic image of the nanotubes.

Fig. 3. X-ray diffraction pattern (a) and Raman spectrum (b) of theHfO2:RE nanotubes. RE, rare earth.

2142 Journal of the American Ceramic Society—Liu et al. Vol. 94, No. 7

coherence that is caused by the nanocrystals and defects in thesamples.

(2) Realization of White Light Emission

Because white light can be formed by appropriate combinationof blue, green, and red light, the white light emission can beobtained by tuning the concentration of the RE ions in the hostmaterial. In order to achieve this goal, blue, green, and red lightemissions should be obtained first. Fortunately, the undoped,Tb-doped, and Eu-doped HfO2 nanotubes can achieve this pur-pose. The corresponding PL spectra are shown in Fig. 4. Theundoped HfO2 nanotubes show intense blue emission. Thisbroad blue emission centered at around 425 nm can be ascribedto the presence of defect states (especially oxygen vacancies) inHfO2.

24–26 The PL spectrum of HfO2:Tb nanotubes covers blueand green emission while the spectrum of HfO2:Eu nanotubescovers blue and red emission. The blue emission in HfO2:Tb andHfO2:Eu nanotubes can also be attributed to the presence ofdefect states in HfO2. The green emission arises from the inner4f shell transitions of Tb31 ions,27,28 and the red emissionoriginates from the inner 4f shell transitions of correspondingEu31 ions incorporated inside the HfO2 host matrix.29,30 TheseRE31 emissions can be efficiently excited via the fundamentalabsorption of HfO2.

2,4 In the PL spectrum of HfO2:Tb nano-tubes, the four characteristic peaks centered at 492, 548, 590,

and 624 nm can be assigned to the 5D4-7F6,

5D4-7F5,

5D4-7F4, and

5D4-7F3 transitions of Tb

31 ions, respectively.The strongest emission peak at 548 nm is responsible for thetypical Tb31 ion green light emission of the 5D4-

7F5 transition.The PL spectrum of HfO2:Eu nanotubes annealed at 5001Ccorrespond to the 5D0-

7FJ (J5 0–2) transitions of Eu31 ions.The five emission peaks centered at 581, 592, 600, 616, and 630nm are assigned to the 5D0-

7F0,5D0-

7F1,5D0-

7F1,5D0-

7F2, and5D0-

7F2 transitions, respectively. The strong-est emission peak at 616 nm is responsible for the characteristicEu31 ion red light emission of the 5D0-

7F2 transition. The lu-minescent spots of the samples are inserted in Fig. 4. The threespots correspond to the undoped HfO2, HfO2:Tb

31, andHfO2:Eu

31 nanotubes, respectively. It is found that the blue,green, and red light emissions are strong enough to be easilyviewed with the naked eye.

Then, different sputtering area ratios of Eu2O3/Tb/Hf sam-ples were prepared to produce white light emission. During thesputtering, we fixed the quantity of Tb while only changingthe area ratio of Eu2O3 flakes on the Hf target to fine tunethe elemental concentration in the samples. The molar ratios(Eu/Tb/Hf) measured by ICP-OES were 0.94/5.85/93.21, 1.37/5.72/92.91, and 1.69/6.08/92.23, and the three samples werenominated as samples B, C, and D, respectively. The PL spec-tra of these samples annealed at 5001C are shown in Fig. 5(a). Itcan be seen that the emission bands of all the HfO2:Eu&Tbnanotube samples cover the RGB primary colors, which corre-spond to the defect states and RE ions emissions in HfO2 host asmentioned in Fig. 4. The red emission band intensity increasesrelative to the blue emission with increasing Eu ratio, while theother emission bands are almost unchanged. Therefore, whitelight emission could be achieved by adjusting the concentrationsof the RE ions in the HfO2 host matrix. The PL spectra of thesamples were converted to the Commission International deI’Eclairage (CIE)-1931 chromaticity diagram and are presentedin Fig. 5(b). It shows that near white light emissions (points B,C, and D) are very close to the ideal chromaticity coordinatesfor pure white light (0.333, 0.333) (point A), the luminescencecolor can be changed from cold white light to warm white lightby changing the molar ratio of Eu/Tb/Hf. The CIE coordinatesand the Tc of the samples are listed in Table I. It is worth notingthat good CIE coordinates (0.333, 0.323) and Tc (5465 K) areobtained with white light emission in sample C.

(3) Effect of Morphology on Luminescence Enhancement

To further investigate the effect of morphology on the lumines-cence properties of RE-doped HfO2 nanotubes, undoped and

Fig. 4. Photoluminescence spectra of the undoped, Tb-doped, and Eu-doped HfO2 nanotubes. The inset (right) shows the luminescent spotscorresponding to the light emissions of the samples, respectively.

Fig. 5. (a) Photoluminescence spectra of the HfO2:Eu31&Tb31 nanotubes with various molar ratios. (b) CIE coordinate diagram showing the chro-

maticity points of the samples with different molar ratios: sample B (blue), sample C (green), and sample D (red). CIE coordinate of the samples are veryclose to the standard equal energy white light illuminate (point A). CIE, Commission International de I’Eclairage.

July 2011 Rare Earth-Doped HfO2 Nanotubes 2143

RE-doped HfO2 films were also prepared using the same sput-tering process. PL spectra of the samples are depicted in Fig. 6.There are no significant emissions in the film samples, while thelight-emission intensities of the HfO2 nanotube samples becomevery strong. Particularly, the defect-related blue emission peakof the undoped HfO2 nanotubes is much higher than that of thefilm. The biggest difference between the film and nanotube sam-ples is that nanotubes have a much higher surface-to-volumeratio. The surface breaks chemical bonds and lattice long-rangeperiodicity. It receives defects and contaminants while segregat-ing from the bulk during growth as well as contaminants ad-sorbed onto it after growth, leading to more surface states in thenanotubes. Therefore, a large number of surface states arisefrom the nanotubes, and these high-density surface states emitstrong blue luminescence. This surface luminescence effect hasbeen proved in many reports.31–33 In Fig. 6, comparing theHfO2:Eu&Tb film and nanotubes, the three RGB light-emissionintensities of the HfO2:Eu&Tb nanotubes increases dramati-cally, indicating that the high-density surface states not onlyemit strong surface state-related luminescence but also greatlyenhance PL properties of RE ion-doped nanostructures. Thedefect states transfer excitation energy to the adjacent RE ionsites and thus greatly increase the emissions.32,34 Consequently,the HfO2:Eu

31&Tb31 nanotubes emit white light motivated byhigh density of defect states (especially surface states), suggest-ing that these nanotubular structures can efficiently enhancelight emission in RE-doped HfO2.

The nanotubular HfO2 host acts as an excellent medium forefficient energy absorption and transfer as mentioned above.Several models have been reported to explain the energy-transfermechanism between the metal oxide hosts and the incorporatedRE impurities.35–38 The results in our research have shown thatthe energy-transfer process can be explained by the defect-related Auger transition model.37,38 In the HfO2 nanotubes, de-fect states mostly arise from the abundant surface states. Such alocalized state at around 2.92 eV (425 nm) is located at almostmidgap. Therefore, after excitation by a 325 nm laser, electronscan be excited from the valence band to the defect level or fromthe defect level to the conduction band by light absorption, andthen the deexcitation transitions of these excited e–h pairs will beaccompanied by radiative emission and nonradiative energy

transfer. The blue emission might be derived from the radiativeemission. Meanwhile, some energy might be quenched non-radiatively by energy transfer to other defects or throughelectron–phonon interactions. At the same time, in the existenceof surrounding RE ions, part of the nonradiative energy will beeffectively transferred to RE ions through Auger energy transi-tion to enhance the RE ion PL emissions. Therefore, the Augerprocess might associate with deexcitation of the excitons cap-tured in the defect states and excitation of RE31 ions simulta-neously occurs through nonradiative transition; thus, HfO2

nanotubes are efficient hosts to absorb and transfer energy toRE31 ions to develop the various emissions.

IV. Conclusions

RE-doped HfO2 nanotubes have been prepared using a sput-tering technique with PVP nanofibers as templates. Theannealed nanotubes maintained a uniform structure. PL mea-surements showed that blue, green, and red emissions could beobtained from the defect states, 5D4-

7FJ (J5 3–6) transitionsof Tb31 ions, and 5D0-

7FJ (J5 0–2) transitions of Eu31 ions inHfO2 shells, respectively. After fine tuning the concentration ofRE ions in HfO2:Eu&Tb nanotubes, white light emission withCIE coordinates of (0.333, 0.323) and Tc of 5465 K wasachieved. In comparison with the films, HfO2:Eu&Tb nano-tubes showed intense white light emission, indicating that theHfO2 nanotubes are efficient hosts to develop RE ion lumines-cence. This observable PL property could be ascribed to thehigh density of surface states in HfO2 nanotubes. Accordingly,the energy-transfer mechanism in the HfO2 nanotubes couldbe explained by the defect-related Auger transition model.Such good white light emission from the HfO2:Eu

31&Tb31

nanotubes makes them a relevant candidate in white lightnanodevices.

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