Why sound and icon is not working on firebase andr

问题:

I successfully set push notification by changing of realtime database.For this i used below function to get notification when app is in background. But the problem is whenever notification is arrived there is no sound play and no icon display. There is no any coding of notification in client side ,Here is my function:

'use strict'
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);


exports.sendNotification = functions.database.ref('/Notifications/{receiver_user_id}/{notification_id}')
.onWrite((data, context) =>
{
    const receiver_user_id = context.params.receiver_user_id;
    const notification_id = context.params.notification_id;


    console.log('We have a notification to send to :' , receiver_user_id);


    if (!data.after.val()) 
    {
        console.log('A notification has been deleted :' , notification_id);
        return null;
    }

    const DeviceToken = admin.database().ref(`/Users/${receiver_user_id}/device_token`).once('value');

    return DeviceToken.then(result => 
    {
        const token_id = result.val();

        const payload = 
        {
            notification:
            {
                title: "New Chat Request",
                body: `you have a new Chat Request, Please Check.`,
                icon: "default",
                sound: "default"
            }
        };

        return admin.messaging().sendToDevice(token_id, payload)
        .then(response => 
            {
                console.log('This was a notification feature.');
            });
    });
});