Luminescence Modulation of Ordered Upconversion

Luminescence Modulation of Ordered Upconversion

Luminescence Modulation of Ordered Upconversion Nanopatterns by a Photochromic Diarylethene: Rewritable Optical Storage with Nondestructive Readout

By Chao Zhang, Huan-Ping Zhou, Long-Yan Liao, Wei Feng, Wei Sun, Zhan-Xian Li, Chun-Hu Xu, Chen-Jie Fang, Ling-Dong Sun, Ya-Wen Zhang, and Chun-Hua Yan*

Optical storage materials,[1] one category of the most promising recording media in this digital age, are being extensively and intensively studied in pursuit of higher data densities. For two-dimensional (2D) recording media, the high-density regime requires each single data bit to be recorded within a reduced area, and simultaneously, it also requires the recording material to exhibit high uniformity at small scales with a precise periodicity over large scales so as to accommodate the digital bits.[2] Besides the widely studied atomic and molecular lattices, superlattices consisting of monodisperse inorganic nanoparticles (sometimes referred to as ‘‘artificial atoms’’) may also afford ideal periodic patterns.[3,4] In recent years, these nanoparticles have been assembled into 2D ultrathin films covering macroscopic areas by the ‘‘bottom-up’’ approach.[3,4a,4b] In contrast to the conventional top-down method, the bottom-up approach allows films to be fabricated by ‘‘soft’’ methods under mild conditions, which is of great technological importance for their application in sensors,[5] catalysis,[6] electronics,[7] photonic devices,[8] and also data storage.[1b,9] It can be envisaged that these ‘‘artificial atoms’’, by virtue of their narrow size distribution, if assembled into finely ordered 2D nanopatterns, would display excellent long-range periodicity. These appealing features ideally cater to the demands of 2D digital storage media, where the data bits are periodically arranged. In this regime, data bits with nanometer or even subnanometer size are pursuable, whereupon ultrahigh recording density can be promised.[9]

Guided by this aim, we have devoted ourselves to the construction of a novel optical storage medium based on the orderly assembled nanopatterns of monodisperse nanoparticles. Rare-earth-doped upconversion nanophosphors (UCNPs) were employed as the structural units of the recording film, for they can be excited by nondestructive near-infrared (near-IR) illumination,[1h] and the resulting photoluminescence (PL) serves as a very sensitive readout window.[1b j] In order to further endow the film with photoswitchable features, other photosensitive species can be introduced for extrinsic functional integration. Organic photochromic materials, especially diarylethenes, by virtue of their flexible molecular structure, reversible photoisomerization properties, and variable spectral profiles, allow the data bits to be rewritable.[10] So we speculated that with a robust diarylethene derivative as a PL modulator, the ordered inorganic luminescent nanopatterns would provide rewritable digital storage functionalities, while the high-data-density potential is still retained by the periodically arranged UCNPs. Hence, in this Communication, we present a prototype of a rewritable 2D optical storage medium with potential high-density recording capacity that functions by the luminescence modulation of ordered upconversion nanopatterns via a photochomic diarylethene.

Monodisperse b-NaYF4:Yb,Er UCNPs were selected to compose the inorganic photoluminescent layer.[1] Upon near-IR excitation (l¼980nm), they exhibit green and red luminescence bands with maxima at ca. 540 and 650nm, respectively (Fig. 1). For the organic photochromic layer, a diarylethene derivative 1-{4-(5-methoxy-2-(2-pyridyl)thiazolyl)}-2-{3-(2-methylbenzo[b]- thiophenyl)}hexafluorocyclopentene (1) was selected,[12] which exhibits reversible transformation between the open-ring isomer (1O) and the closed-ring isomer (1C) under conditions of appropriate UV and visible irradiation (Scheme 1). Notably, in contrast to 1O, 1Cdisplays a broad absorption band centered at ca. 550nm (Fig. 1), which ideally overlaps with the narrow emission band of the inorganic phosphors around 540nm, suggesting that the PL intensity of UCNPs can be modulated by controllable energy transfer between the inorganic and organic species.[1h]

The composite film was fabricated by sequential deposition of the inorganic and organic species onto a silicon wafer. To prepare the inorganic layer, three facile ‘‘bottom-up’’ assembly methods were adopted: spin-coating (SC), the Langmuir–Blodgett (LB) technique, and evaporation-induced self-assembly (EISA). Under optimized conditions, large-scale continuous film (on at least

[*] Prof. C.-H. Yan, C. Zhang, H.-P. Zhou, L.-Y. Liao, Dr. W. Feng,

Dr. W. Sun, Dr. Z.-X. Li, C.-H. Xu, Dr. C.-J. Fang, Prof. L.-D. Sun, Prof. Y.-W. Zhang Beijing National Laboratory for Molecular Sciences State Key Laboratory of Rare Earth Materials Chemistry and Applications PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry Peking University Beijing 100871 (P. R. China) E-mail:

DOI: 10.1002/adma.200901722

TION millimeter scale) was obtained with each of the three methods. The morphologies of the as-prepared films were examined by scanning electron microscopy (SEM). The SC film consists of randomly distributed nanoparticles (typically 20nm in diameter), yet still can be considered a uniform film on the micrometer scale (Fig. S1a,b in the Supporting Information). The LB film is a monoparticulate layer exhibiting a much more ordered superstructure, with some defects though (Fig. 2a,b), and the EISA film displays a highly ordered nanopattern of particles over micrometers (Fig. 2c,d). The morphology differences between these films can be explained in terms of the mobility of nanoparticles and the temporal duration of particle relaxation in the assembly process. For example, in the EISA case, the nanoparticles in the organic solvent are highly mobile (in contrast to the nanoparticles constrained at the air/water interface in the LB case) and the evaporation rate is relatively slow (in contrast to the fast evaporation in the SC case), so the particles tend to form well-ordered nanopatterns. In addition, it is noteworthy that all procedures here were performed at room temperature under ambient atmosphere; no rigorous conditions were required.

The organic species was further deposited by dispensing ethanol solution of 1 onto the UCNP-deposited silicon wafers and allowing the solvent to evaporate at room temperature. The stability of the inorganic layers during this procedure was examined by control experiments (see the Experimental section and Supporting Information), which confirmed that the morphology of the inorganic film is not essentially altered after the deposition of the organic constituent (Fig. S2). This is probably owing to the poor dispersibility of the UCNPs in ethanol.[11c]

As no essential difference was revealed between the three films on the micrometer scale (the same scale as the focus diameter of our optical apparatus), we chose to use the LB-based film for further research into data recording performance, considering the simple scale-up capability and high-throughput feature of LB fabrication.[3]

In the data recording experiments, the composite film sample was placed in the spectrometer, illuminated at grazing angle by the near-IR laser beam (l¼980nm), and the PL spectrum recorded accordingly (Fig. 3a). An emission band was detected around 540nm, with a maximal intensity normalized as 1.0. Then a UV laser beam (l¼325nm) was focused on the sample, triggering the conversion from 1O to 1C. Consequently, a decreased PL intensity of 0.09 was detected around 540nm, indicating that the emission from the UCNPs was quenched by 1C. Subsequently, a visible-light beam was conducted onto the sample, and the PL emission was finally recovered, from which we infer that the photo-cycloreversion from 1C to 1O restored the initial luminescence. During the whole photoswitching process, the high/low intensity ratio (namely, the ‘‘1’’ to ‘‘0’’ ratio) at 540nm is estimated to be ca. 1. Seven cycles of alternating UV/ vis irradiation were performed (Fig. 3b), and the PL switching operations were found to be well repeatable, implying no obvious degradation of the recording material. This is ascribable to the excellent thermo-/photostability of all the species employed. In addition, it is noteworthy that the PL peak around 650nm is barely influenced by the UV/vis irradiation, owing to the negligible absorbance of both 1O and 1C in this region.

The destruction of data bits during the readout process was also investigated (Fig. 3c). During 1500 s of near-IR illumination, neither ‘‘1’’ nor ‘‘0’’ signal intensity exhibits significant change. The readout non-destructivity is attributed to the unique reading excitation wavelength employed here.[1h] In the PL readout mode,

Figure 1. Absorption spectra of the open-ring form and closed-ring form of diarylethene 1 (dissolved in acetonitrile, high performance liquid chromatography (HPLC) grade, photocyclization achieved by irradiation at 325nm) and emission spectrum of NaYF4:Yb,Er UCNPs (deposited on silicon wafer, excited at 980nm).

Scheme 1. Molecular structures of the open-ring and closed-ring isomers of the diarylethene 1 and the corresponding photoisomerization reactions.

Figure 2. a,b) SEM images of LB film consisting of NaYF4:Yb,Er UCNPs on different scales. c,d) EISA film on different scales (the EISA images are a little blurred, probably due to the presence of a certain amount of organic surfactant, which is necessary to obtain the ordered nanopattern).

2 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–5 Final page numbers not assigned

destructive readout usually results from the photoisomerization of the modulators triggered by the reading beam excitation (usually in the UV or visible range)[1a,e] but in the present system, the reading wavelength is located in the near-IR region, where photons have lower energy. Therefore, for either 1O or 1C,a near-IR photon is not able to promote an electronic transition between frontier molecular orbitals; hence, neither 1O nor 1C shows considerable absorbance in this region and, thus, no photoisomerization is possible upon near-IR illumination. As a result, the risk of destructive readout is practically obviated. In fact, the near-IR-illumination strategy qualifies many other molecular photochromes as the organic modulator in this regime, for under conventional conditions photochromic molecules rarely respond to near-IR excitation, on condition that their p-conjugation systems are not extraordinarily extended.

Notably, it is the cooperation of the inorganic and organic components that enables the exploitation of near-IR excitation, because the inorganic UCNPs and the organic molecules show distinct excitation properties. Specifically, under single-beam excitation, an inorganic UCNP is multiphoton absorptive, and ‘‘collects’’ two near-IR photons to yield one visible photon,[1,13] whereas an organic molecule is normally single-photon absorptive, and accordingly ‘‘ignores’’ the near-IR excitation. Therefore, during the readout process, the inorganic UCNPs keep luminescing upconversion photons, while the organic photochromes remain essentially intact.

By employing the ‘‘UCNP–photochrome’’ regime discussed above, we constructed a composite film medium capable of digital recording. A typical demonstration is presented in Figure 4. In the initial state, the diarylethene layer was in the 1O form, and intense green emission was detected within the observed window with an area of 30mm 30mm (Fig. 4a). When UV irradiation was introduced, the original intense green emission was quenched, corresponding to the ‘‘erasing’’ operation (Fig. 4b). Then four data bits in a square configuration were ‘‘rewritten’’ on the sample by local irradiation with visible light (Fig. 4c), and practically no destruction was observed during the next 3600s near-IR readout process (Fig. 4d). In addition, the defects in the LB film do not compromise the data-storage performance in practice, probably because the size of each defect point (typically 20 30nm in diameter) is much smaller than the optical diffraction limit.

Limited by the focus size (typically 10mm in diameter) of our instruments, the data density of the storage medium above is estimated to be 6.15Mb in. 2 (ca. 0.95Mb cm 2), whereas intrinsically it is the uniformity of the composite film, in particular the periodicity of the inorganic film, that dictates the ultimate areal data-storage capacity.[2] As discussed above, the EISA films exhibit finely ordered morphological nanopatterns, so the storage capacity could be further increased. For example, the EISA film in Figure 2c consists of monodisperse UCNPs with precisely periodic superstructure. Assuming that each single particle can be used for 1bit of digital data, with a 2D compact hexagonal superlattice, the potential data density is immensely enhanced to 1.47Tb in. 2 (in association with optical superresolution methodologies, such as scanning near-field optical microscopy (SNOM)).[1i,14] In other words, a high density of recorded digital data can be promised by spatially well-arranged periodic nanopatterns.

In conclusion, a prototypical 2D rewritable optical storage medium has been successfully constructed by the rational assembly of inorganic and organic materials. With diarylethene derivative 1 as the photochromic modulator, rewritability and durability of the recording medium is guaranteed. Further, by

Figure 3. a) Photoswitching behavior of the composite film from initial state to PL-decreased state and back to the retrieved state. b) PL intensity at 539nm recorded for 7 cycles of alternating UV/vis irradiation. c) PL intensity (average integration from 539nm to 540nm) versus reading time of the composite film, during 1500s of continuous illumination at 980nm.

TION virtue of the unique excitation property of b-NaYF4:Yb,Er UCNPs, exploitable readout wavelengths are extended to the near-IR region, where neither 1O nor 1C shows any observable absorbance, so the problem of destructive readout is circumvented. Also, this inorganic/organic binary system can be extended to other UCNPs and photochromes whose spectra show considerable overlap (e.g., red-emissive a-NaYF4:Yb,Er UCNPs[11c] and red-absorptive spiropyrans[1a] or diarylethe- nes[1e]). Notably, the writing, erasing, and reading of data bits are all performed under single-beam irradiation on 2D recording media, suggesting the compatibility of this strategy with current CD/DVD drivers. Athough multiwavelength light sources are required in this system and several other ‘‘fluorophore– photochrome’’ storage systems,[1e,f] the afforded PL emission in the meanwhile provides a readily readable window with the advantages of low background and high sensitivity, which are of particular importance when super-resolution techniques are employed.[1i] Moreover, by taking advantage of the soft-assembly behaviors of monodisperse UCNPs, continuous films with different degrees of order have also been fabricated by spin-coating, LB, and EISA techniques, respectively. The monolayer feature and good periodicity of the EISA film implies an ideal thickness, roughness, and uniformity, which has encouraged us to carry out experimentation by SNOM so as to pursue higher data density. On the other hand, the fabrication of 3D assemblies of nanoparticles[4a] may also shed light on 3D optical storage using this UCNP-photochrome strategy if the energy transfer can be controlled. Relevant research is under way in our group.

Experimental b-NaYF4:Yb,Er nanoparticles (typically 20nm in diameter) were synthesized as previously described [11c]. These UCNPs were separated by centrifugation after the addition of ethanol, and redispersed in nonpolar solvents.

Diarylethene 1 was synthesized and purified as previously described [12]. The absorption spectra were recorded on a UV-3100 spectrophotometer (Shimadzu). The spin-coated film was prepared from a colloidal solution of

NaYF4:Yb,Er nanoparticles (ca. 10mmol L 1) in chloroform (A. R.). The film was cast on a silicon wafer (20mm 10mm) at 800rpm.

The LB film was fabricated on a LB trough (Nima Technology, M611) from a colloidal solution of NaYF4:Yb,Er nanoparticles (ca. 1mmol L 1)i n chloroform (A. R.). The colloidal solution (500 mL) was spread onto the ultrapure water surface. After the solvent had evaporated, the monolayer was compressed at a constant speed of 5cm2 min 1 from 259.0cm2 to 31.0cm2 with a corresponding surface pressure of 35mN m 1.

Subsequently the monoparticulate layer on the air/water interface was carefully transferred onto a silicon wafer (10mm 10mm) using the Langmuir–Schaefer horizontal liftoff procedure.

The EISA film was prepared from a colloidal solution of NaYF4:Yb,Er nanoparticles (ca. 50mmol L 1) in cyclohexane (A. R.). One drop of the solution (50 mL) was dispensed on a silicon wafer (5mm 5mm) and the solvent was evaporated at room temperature.

Diarylethene 1 (10mg) was dissolved in ethanol (1mL) and one drop of the solution (4 mL) was dispensed onto the silicon wafer where the LB film had been deposited. Solvent was evaporated at room temperature.

Data recording experiments were performed on a HR-UV-800 spectrometer (Horiba Jobin Yvon), in association with an external tunable 2W near-IR diode laser (l¼980nm). The composite film sample was exposed to ambient light for 24h to ensure 1C transformed fully into 1O. Then the film was placed in the spectrometer, illuminated at grazing angle by the near-IR laser beam, and the PL spectrum detected accordingly. In the writing procedure, a UV laser beam (l¼325nm) from the spectrometer was focused on the sample for 10min, whereas in the erasing procedure a focused beam from an incandescent lamp was conducted onto the sample for 10min.


The authors thank the NSFC (Nos. 20821091, 20671005, and 20731160001) and MOST of China (2006CB601104) for financial support. Supporting Information is available online from Wiley InterScience or from the authors.

Received: May 23, 2009 Published online:

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Figure 4. PL mapping images of the composite film illustrating a) initial state, b) erasing, c) rewriting, and d) nondestructive readout under 980nm excitation for 3600s.

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