Changjian Zhou

Graphene-on-Paper Sound Source Devices

We demonstrate an interesting phenomenon that graphene can emit sound. The application of graphene can be expanded in the acoustic field. Graphene-on-paper sound source devices are made by patterning graphene on paper substrates. Three graphene sheet samples with the thickness of 100, 60, and 20 nm were fabricated. Sound emission from graphene is measured as a function of power, distance, angle, and frequency in the far-field. The theoretical model of air/graphene/paper/PCB board multilayer structure is established to analyze the sound directivity, frequency response, and efficiency. Measured sound pressure level (SPL) and efficiency are in good agreement with theoretical results. It is found that graphene has a significant flat frequency response in the wide ultrasound range 20-50 kHz. In addition, the thinner graphene sheets can produce higher SPL due to its lower heat capacity per unit area (HCPUA). The infrared thermal images reveal that a thermoacoustic effect is the working principle. We find that the sound performance mainly depends on the HCPUA of the conductor and the thermal properties of the substrate. The paper-based graphene sound source devices have highly reliable, flexible, no mechanical vibration, simple structure and high performance characteristics. It could open wide applications in multimedia, consumer electronics, biological, medical, and many other areas.

Transparent, flexible, ultrathin sound source devices using Indium Tin oxide films

Thermoacoustic effects were observed in 100-nm indium tin oxide (ITO) films. The sound emission from the ITO films was measured as a function of power, distance, and frequency. Significant flat and wide frequency responses occurred between 20 and 50 kHz. The sound pressure and efficiency were in good agreement with theoretical results. This indicates that a thermoacoustic effect exists in metal-oxide materials and that a large family of transparent electrode materials may exhibit similar properties. Using the ultrathin, transparent, and flexible characteristics, we showed promising applications of ITO sound source devices that were integrated with liquid crystal display screens.

Flexible, ultrathin, and transparent sound-emitting devices using silver nanowires film

We demonstrated flexible, ultrathin, and transparent sound-emitting devices (SEDs) using silver nanowires (AgNWs). Large area of AgNWs film on substrate was made by dry transfer technique. The sound emission from the AgNWs was measured as a function of power, distance, and frequency. Significant flat and wide frequency responses occurred between 15 and 45 kHz. The sound pressure was in good agreement with theoretical results. This indicates that a thermoacoustic effect exists in AgNWs. The AgNWs-SEDs can be integrated with the liquid crystal display, which shows the potential to be an important component in flexible electronic systems.

Resistive switching behavior in diamond-like carbon films grown by pulsed laser deposition for resistance switching random access memory application

In this paper, nonvolatile bipolar resistive memory effects were observed in nitrogen doped diamond-like carbon (DLC) thin films prepared by a pulsed laser deposition technique. It is observed that the fabricated Pt/Ti/DLC/Pt structure exhibits good memory performances with an ON/OFF ratio >10, data retention time >104 s, and low operation voltage (<1.5 V). The current mechanism is fitted by Ohmic and space charge limited conduction laws in low resistance state and high resistance state scenarios. The formation/rupture of metal filaments is due to the diffusion of the titanium ions.

Temperature-Compensated High-Frequency Surface Acoustic Wave Device

We report high-frequency surface acoustic wave (SAW) devices with excellent temperature stability using a layered structure consisting of single-crystal LiNbO3 thin film on SiO2/LiNbO3 substrate. SAW devices with a wavelength of 2 μm have been fabricated and several wave modes ranging from ~ 1.5 to 2.1 GHz have been obtained. With the SiO2 interlayer providing the temperature compensation and the top single-crystal Z-cut LiNbO3 piezoelectric thin film for acoustic wave excitation, the fabricated SAW devices exhibit excellent temperature coefficients of frequency. Theoretical calculations are presented to elucidate temperature compensation of the proposed layered structure.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)-based organic, ultrathin, and transparent sound-emitting device

An organic, ultrathin, and transparent sound-emitting device was fabricated using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic) (PEDOT:PSS) films. This sound-emitting device was easily processed by spin coating. Sound emission from PEDOT:PSS was measured as a function of power, distance, and frequency. The sound frequency spectrum was found to be ultra-flat in a wide sound frequency range (10–45 kHz). Theoretical analysis showed that the working principle was the thermoacoustic effect. The PEDOT:PSS sound-emitting device has potential applications in the acoustic field. In particular, it can be integrated with a liquid crystal display to realize sound emission and image display at the same time.

Static behavior of a graphene-based sound-emitting device

Due to the extremely high thermal conductivity and low heat capacity per unit area of graphene, it is possible to fabricate an efficient sound-emitting device based on the thermoacoustic effect with no mechanical vibration. In this paper, the fundamental performance of this new graphene sound-emitting device (G-SED) is investigated in terms of its static behavior. The sound amplitude mapping shows that the G-SED has good sound performance under 0.01 W. The sound frequency spectra measured at different distances and angles show that the G-SED has good sound directivity. It is possible to realize sound wave manipulation by using an array of G-SEDs. The relationship between the temperature of graphene and the sound frequency was investigated by a thermal imaging instrument. The fast transient sound response in real time was recorded by applying 60 μs short time multi-pulses and single-pulse. The stable sound emission at a constant sound pressure amplitude with low noise was observed for continuous operation under a fixed frequency over several hours. Such significant performances in this G-SED indicate broad applications, and shed light on the use of graphene in the field of acoustics.

High-density pMUT array for 3-D ultrasonic imaging based on reverse-bonding structure

High-density piezoelectric micromachined ultrasonic transducer (pMUT) arrays have been designed and fabricated using silicon based micromachined process. These devices are based on a novel structure called “reverse-bonding”. Experimental results show that they have excellent performance, suitable resonance frequency and high reliability. The fabricated pMUT arrays are very promising for 3-D medical imaging.

Visible-light photoresponse of AlN-based film bulk acoustic wave resonator

Visible-light photoresponse of an AlN-based film bulk acoustic wave resonator (FBAR) is demonstrated. It is found that the FBAR exhibits a resonant frequency downshift under purple light illumination and the magnitude of the frequency downshift increases as the power density increases within the range of 5–40 mW/cm2. A resonant frequency downshift of 1313 KHz is observed under 40 mW/cm2 illumination, corresponding to a minimum detection power of 6.09 nW. A sub-bandgap photoresponse of the AlN thin film is proposed to explain this phenomenon.

Flexible structured high-frequency film bulk acoustic resonator for flexible wireless electronics

Flexible electronics have inspired many novel and very important applications in recent years and various flexible electronic devices such as diodes, transistors, circuits, sensors, and radiofrequency (RF) passive devices including antennas and inductors have been reported. However, the lack of a high-performance RF resonator is one of the key bottlenecks to implement flexible wireless electronics. In this study, for the first time, a novel ultra-flexible structured film bulk acoustic resonator (FBAR) is proposed. The flexible FBAR is fabricated on a flexible polyimide substrate using piezoelectric thin film aluminum nitride (AlN) for acoustic wave excitation. Both the shear wave and longitudinal wave can be excited under the surface interdigital electrodes configuration we proposed. In the case of the thickness extension mode, a flexible resonator with a working frequency as high as of 5.2325 GHz has been realized. The resonators stay fully functional under bending status and after repeated bending and re-flattening operations. This flexible high-frequency resonator will serve as a key building block for the future flexible wireless electronics, greatly expanding the application scope of flexible electronics.